Implement ProcessMonitor::Kill for Linux

On FreeBSD ptrace(PT_KILL) is used to terminate the traced process
(as if PT_CONTINUE had been used with SIGKILL as the signal to be
delivered), and is the desired behaviour for ProcessPOSIX::DoDestroy.

On Linux, after ptrace(PTRACE_KILL) the traced process still exists
and can be interrogated.  It is only upon resume that it exits as though
it received SIGKILL.

As the Linux PTRACE_KILL behaviour is not used by LLDB, rename
BringProcessIntoLimbo to Kill, and change the implementation to simply
call kill() instead of using ptrace.

Thanks to Todd F for testing (Ubuntu 12.04, gcc 4.8.2).

Sponsored by: DARPA, AFRL
Differential Revision: http://llvm-reviews.chandlerc.com/D3159

llvm-svn: 205337