We don't care, at the process level, whether or not we're single
stepping. That state is really only relevant at the thread level.

Breakpoints, tracepoints, etc.. take a location spec as input. This
patch improves the expressiveness of that API. It allows:

* Breakpoint at line
* Breakpoint at function (handling package / receiver smoothing)
* Breakpoint at address
* Breakpoint at file:line
* Setting breakpoint based off regexp

the entry point of a function is the beginning of the prologue, which can be run multiple times for each invocation of a function if the stack needs to be expanded or the scheduler needs to be run.

proc.(*Thread).GetG: reading TLS memory directly for g address instead of modifying the executable code

bugfix, Issue #163: offset of g struct in TLS picked based on the value of runtime.buildVersion and presence of compile units created by GNU AS, instead of being fixed to -16

Also, reorganizes some code.

Initially, the `proc` package emitted a lot of output. Now, that should
not be the case. The `proc` package should never print, for any reason.
That should be handled by clients.

We do not need to verify a current breakpoint, nor do a redundant check
on whether we have been asked to manually halt. Assume trapWait has done
its due diligence and stop the world once it returns.

Also, kill whitespace to make code appear more as a singular block for
readability.

Instead of fighting against the normal flow, just signal a SIGTRAP and
let the existing flow handle it, as long as we set the halt flag
correctly the system should halt.

additionally fixes a bug when Detach is called on an exiting/exited thread: dbp.CurrentThread could point to a thread that has already been removed from dbp.Threads by trapWait which will lead to a nil pointer dereference caused proc.Process.clearBreakpoint getting nil from dbp.Threads[tid]

On a thread that's leader of its group,
that is ptraced and that was survived by its children.

Makes for more deterministic test runs.

s/Break/SetBreakpoint/
s/Clear/ClearBreakpoint/
s/BreakByLocation/SetBreakpointByLocation/

Finishes #63 #64

Previously either the terminal client or the debugger service would
either lock main goroutine to a thread or provide a locked goroutine to
run _all_ DebuggedProcess functions in. This is unnecessary because only
ptrace functions need to be run from the same thread that originated the
PT_ATTACH request.

Here we use a specific thread-locked goroutine to service any ptrace
request. That goroutine is also responsible for the initial spawning /
attaching of the process, since it must be responsible for the PT_ATTACH
request.