ProcessMonitor improvements for Linux.
-Remove tracing of fork/vfork until we add support for tracing inferiors' children on Linux.
-Add trace exec option for ptrace so that we don't receive legacy SIGTRAP signals on execve calls.
-Add handling of SIGCHLD sent by kernel (for now, deliver the signal to the inferior).


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@182153 91177308-0d34-0410-b5e6-96231b3b80d8