After recent changes we weren't reaping child processes resulting in many
zombie processes. 

This was fixed by adding more settings to the ProcessLaunchOptions class
that allow clients to specify a callback function and baton to be notified
when their process dies. If one is not supplied a default callback will be
used that "does the right thing". 

Cleaned up a race condition in the ProcessGDBRemote class that would attempt
to monitor when debugserver died. 

Added an extra boolean to the process monitor callbacks that indicate if a
process exited or not. If your process exited with a zero exit status and no
signal, both items could be zero.

Modified the process monitor functions to not require a callback function
in order to reap the child process.

llvm-svn: 144780