Fixes #262

Breakpoints are skipped either because:
1. when multiple breakpoints are hit simultaneously only one is
processed
2. a thread hits a breakpoint while another thread is being
singlestepped over the breakpoint.

Additionally fixed a race condition between Continue and tracee
termination.

Prevents a lot of goroutines hanging around, especially when running
tests.

/proc/pid/stat needs more complex parsing

Fixes #239

Fixes a bug on OSX where, if the debugged process spawned a child, when
that process received a SIGCHLD it would cause Delve to hang.

Fixes #197

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.

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.

Fixes #154

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

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.

Process is an incorrect name for the DebuggedProcess struct that the
thread is "a part" of. Also, no need to export that field.

This information is hardly going to be useful to a user of Delve and if
the user really needs information on new threads they can simply using
the provided commands to list and switch between threads.

* Ignore signals we do not care about
* Implement custom fork/exec for Darwin to convert signals to mach
  exceptions