GDBSERVER: Listen on a unix domain (instead of TCP) socket if requested.

GDBSERVER: Listen on a unix domain (instead of TCP) socket if requested.

When invoking gdbserver, if the COMM parameter takes the form "unix::/path/name"
then a local (unix) domain socket will be created with that name and gdbserver
will listen for connections on that.

    gdb/
    * NEWS: Mention new feature.

    gdb/gdbserver/
    * configure.ac (AC_CHECK_HEADERS): Add sys/un.h.
    * configure: Regenerate.
    * remote-utils.c (remote_prepare): Create a local socket if requested.
     (remote_open):  Don't attempt to open a file if it's a socket.
     (handle_accept_event): Display the name of the socket on connection.

   gdb/common/
   * netstuff.c (parse_connection_spec)[prefixes]: New member for local domain sockets.