Variable lookup is slow because it requires a full scan of debug_info
to check for package variables, this doesn't matter much in interactive
use but can slow down evaluation of breakpoint conditions
significantly.

Providing benchmark proof for this is hard since this effect doesn't
show for small programs with small debug_info sections.

* proc: fix interaction of RequestManualStop and conditional breakpoints

A conditional breakpoint that is hit but has the condition evaluate to
false can block a RequestManualStop from working. If the conditional
breakpoint is set on an instruction that is executed very frequently by
multiple goroutines (or many conditional breakpoints are set) it could
prevent all calls to RequestManualStop from working.

This commit fixes the problem by changing proc.Continue to exit
unconditionally after a RequestManualStop is called.

* proc/gdbserial: fix ContinueOnce getting stuck on macOS

Fixes #902

Fixes #904

The concrete value of an interface is always stored as a pointer inside
an interface variable. So far we have followed the memory layout and
reported the type of the 'data' attribute of interfaces as a pointer,
however this makes it impossible to distinguish interfaces with
concrete value of type 'A' from interfaces of concrete value of type
'*A'.

With this changeset when we autodereference pointers when the concrete
type of an interface is not a pointer.

* Fix various issues detected by megacheck

I've ran honnef.co/go/tools/cmd/megacheck and fixed a few of the
things that came up there.

* Cleanup using Gogland

Windows doesn't actually have ptrace.

Fixes #778

Fixes #885

When a Go program is externally linked, the external linker is
responsible for picking the TLS offset. It records its decision in the
runtime.tlsg symbol. Read the offset from that rather than guessing -16.

This implementation causes a regression: 1.4 and earlier will no longer
work.

Other debuggers can be instructed to decorate the stacktrace with the
value of SP. Our SP equivalent is the frame offset, since we can add it
to the Stackframe structure without incurring into added costs we
should, so that frontends can use it if they want.

* cmd: include replay command in docs even if rr is not installed.

* path/to/binary is the required argument

Replaces #847

A next/step/stepout command could hit a normal breakpoint, decorated
with a list of variables to evaluate, if that happens the variable
should be evaluated just as if the breakpoint was hit by a continue.

Implementing proc.Process.Running in a thread safe way is complicated
and nothing actually uses it besides tests, so we are better off
rewriting the tests without Running and removing it.

In particular:

* The call to d.target.Running() in service/debugger/debugger.go
  (Restart) can never return true because that line executes while
  holding processMutex and all continue operations are also executed
  while holding processMutex.
* The call to dbp.Running() pkg/proc/native/proc.go (Detach) can never
  return true, because it's only called from
  debugger.(*Debugger).detach() which is also always called while
  holding processMutex.

Since some tests are hard to write correctly without Process.Running a
simpler interface, Process.NotifyResumed, is introduced.

Fixes #830

RequestManualStop will run concurrently with trapWait, since one writes
dbp.halt and the other reads it dbp.halt should be protected by a
mutex.

Updates #830

See https://youtrack.jetbrains.com/issue/GO-3931#comment=27-2224179
for more details

If we don't fill the Len field there will be no way for the user to
distinguish maps we didn't load from empty maps.

The condition that causes waitFast to fail can not happen in addThread
and halt so we don't need to call the slower wait.

Buckets of maps with zero sized value types (i.e. map[T]struct{}) have
zero length value arrays.

A waitreason string that has invalid length (because the G struct is
corrupted or being modified) could cause a crash.

type M struct was never used (as far as I know).
type VariableEval interface was used for a brief period of time during
the refactoring, now both its methods are functions.

The authorization prompt on macOS can take a long time to be
acknowledged by the user, we should keep waiting for a connection as
long as the debugserver instance we launched remains alive.

Before this commit our temp breakpoints only checked that we would stay
on the same goroutine.
However this isn't enough for recursive functions we must check that we
stay on the same goroutine AND on the same stack frame (or, in the case
of the StepOut breakpoint, the previous stack frame).

This commit:
1. adds a new synthetic variable runtime.frameoff that returns the
   offset of the current frame from the base of the call stack.
   This is similar to runtime.curg
2. Changes the condition used for breakpoints on the lines of the
   current function to check that runtime.frameoff hasn't changed.
3. Changes the condition used for breakpoints on the return address to
   check that runtime.frameoff corresponds to the previous frame in the
   stack.
4. All other temporary breakpoints (the step-into breakpoints and defer
   breakpoints) remain unchanged.

Fixes #828

Debugserver does not work as documented, "--" needs to be specified to
pass arguments to the target process (but only if it's an argument that
starts with a dash).

Fixes #839

Fixes #836

You usually want to know the reason for the panic when a panic happens,
it can be printed manually this is a small quality of life improvement.

If CurrentThread isn't running a goroutine SelectedGoroutine can be
nil, do not blindly dereference it.

Fixes #827

Implements #727

While implementing the gdbserial backend everything was changed to call
Detach to "close" a process so that gdbserial could do its clean up in
a single place. However the native implementation of Detach does not
actually kill processes we launched.

Fixes #821

debugserver requires a special option to forward the environment to the target.

Fixes #818

Lldb-server does not currently support windows/amd64 (it's in
development). And delve's native backend seems to work well on windows.
Disable the lldb backend on windows for now.

Fixes #817