llgs: implement --setsid.

The --setsid (-S) option changes the session id for the lldb-gdbserver process.
This is used by tools such as lldb-platform and allows the user to prevent
llgs from being in the same session as a calling terminal session.
This will prevents terminal group control signals from affecting
lldb-gdbserver.

See also:
https://github.com/tfiala/lldb/issues/38

llvm-svn: 212873