Patch from Matt Kopec:

This patch fixes an issue where if lldb fails to attach to a process (ie. invalid pid) on Linux, the process monitor thread gets stuck waiting for a signal from the attach thread, which never comes due to not being signaled. It also implements StopOpThread which is used for both attach/launch cases as I'm not aware of any special handling needed for the attach case. Also, propagate 'Error' from the Detach function instead of using a bool.

llvm-svn: 166055