Changes:
* Modify patty_client_new() to accept NULL as a Unix domain socket
path; when NULL is provided, search for the pattyd(8) socket in
pattyd.sock, followed by the default full socket path,
/var/run/patty/patty.sock
* Modify bin/ax25dump.c, bin/tncd.c to use default socket paths when
calling patty_client_new() by defaulting to NULL when no -s flag
is provided