Remove compatibility code for abstract unix domain sockets.

Abstract sockets were introduced in paraslash-0.5.5 (2015-09-20). For
backwards compatibility, create_local_socket() was modified to create
an abtract socket *and* an ordinary pathname socket so that old audiod
clients (which connect to the pathname socket) would still work.

This patch breaks compatibility by going back to a single socket,
either abstract (Linux) or pathname (all other systems), as determined
in the first call to create_local_socket(). This allows to drop the
"abstract" argument of init_unix_addr() and the "mode" argument of
create_local_socket().

The code in net.c and the callers get quite a bit shorter since it
has to deal with only a single file descriptor and one type of socket.