Another attempt to clean up the e-passwords.c code a bit.

  1) First and foremost, e-passwords.c holds the last use of an EMsgPort
     outside of Camel.  It's used to communicate to a waiting thread when
     an EPassMsg has been processed.  EFlag is a better fit here.

  2) Use a GQueue for the message queue instead of an EDList.  Note the
     new logic in pass_response().  It tags nodes for deletion while
     iterating over the GQueue, and then "expunges" the GQueue.

  3) Use GStaticMutex instead of pthread_mutex_t.

  4) Use GThread instead of pthread_t.

Obviously this should wait until 2.23 development opens up.