[gettler] Fix some misc bugs in libcmyth:
- Change cmyth_rcv_okay() and add cmyth_rcv_match() since the "OK"
  responses can be in either upper or lower case.
- Fix cmyth_conn_connect_pathname() to error out if a connection
  handle is not provided.
- Ensure that calls to select() and recv() are retried if interrupted
  by a signal.