Fixed process kill command.

Killing the inferior was not resuming the inferior after its
pre-death signal to NativeProcessLinux.  This kept the process
in a defunct/zombie state since the final status was not reaped.

This change fixes this: the monitor notifies the delegate when
the pre-death exit signal is received.  In the final waitpid
reaping, a check is made to see if the process was already set
to exited.  If so, it just logs the final exit of the process.
Otherwise, as a failsafe, it sets the exit state and notifies
the delegate (and logs that as well as it really shouldn't happen).

I've also removed code that had the exit process handling in
GDBRemoteCommunicationServer detach from the process.  This is
unnecessary since the process either exited or was killed and
will die properly under control of NativeProcessLinux.

See https://github.com/tfiala/lldb/issues/17