Cleanup: sessiond: rename the sessiond main thread quit pipe

Rename the sessiond main thread 'thread_quit_pipe' as the
'main_quit_pipe' to make it easier to distinguish it from the other
threads quit pipes.

While we're there, adjust some debug statements and internal function
names to distinguish between notifying the quit pipe and actually
stopping the threads.

Change-Id: I9caf81f6d03805658e2470a026b15dde03f2a88b
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>