Add prctl PTRACE authorization for lldb-launched inferior.

Implements https://github.com/tfiala/lldb/issues/43.

A further change is needed.  I will need to setup a pipe-based
solution where the child reads from a pipe that lldb creates
before the fork, used for synchronization.  lldb writes a byte
to the pipe only after llgs attaches successfully to the process.

Also adds the [PATCH] content to lldb-commits@ from earlier today
that implements LLDB_DEBUGSERVER_EXTRA_ARG_* environment variable
support.