Added methods to PlatformLinux to start supporting remote platform connection.

PlatformLinux now derives from ProcessPOSIX.

PlatformLinux now implements much of its functionality like ProcessDarwin.
Some (most?) of those methods should be pushed into PlatformPOSIX.  Will
adjust before upstreaming.

Still not getting lldb-gdbserver to run.  And the test runs use a
ProcessPOSIX rather than a ProcessGDBRemote.