Fix llgs to really use the STDIO launch file actions.

* Adds logging around STDIO redirection.
* Removes the ProcessMonitor-derived code that ignores file actions for PTYs.
* Adds PTY hook-up logic to the PlatformLinux local llgs launch code.

Fixes https://github.com/tfiala/lldb/issues/65 for local llgs.

Note: not upstreaming this until I make sure https://github.com/tfiala/lldb/issues/69
works for remote llgs.