Modify debugserver to follow gdb remote $qC protocol definition.

$qC from debugserver now returns the current thread's thread-id (and, like $?, will set a current thread if one is not already selected).  Previously it was returning the current process id.

lldb will now query $qProcessInfo to retrieve the process id.  The process id is now cached lazily and reset like other cached values.  Retrieval of the process id will fall back to the old $qC method for vendor==Apple and os==iOS if the qProcessInfo retrieval fails.

Added a gdb remote protocol-level test to verify that $qProcessInfo reports a valid process id after launching a process, while the process is in the initial stopped state.  Verifies the given process id is a currently valid process on host OSes for which we know how to check (MacOSX, Linux, {Free/Net}BSD).  Ignores the live process check for OSes where we don't know how to do this.  (I saw no portable way to do this in stock Python without pulling in other libs).