Changes:
* Add a "pending disconnect" state to enum patty_ax25_sock_state
(PATTY_AX25_SOCK_PENDING_DISCONNECT), used when an originating
system sends a DISC frame, pending a UA response
* When handling a UA frame, rather than terminating a socket in the
established state implicitly, check for a "pending disconnect"
state explicitly
Other changes:
* When a remote socket has already been obtained in handle_frame()
in src/server.c, pass that socket to any other method that would
otherwise be looking up the same socket