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.
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]
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.
