bug#19394: 25.0.50; mpc does not handle MPD_HOST=socket

[Mark Oteiza]

Hi,

mpc.el treats MPD_HOST only as an address or hostname, when it can also
be a socket. So, doing ‘M-x mpc RET’ here gives

    /run/user/1000/mpd/socket/6600 Name or service not known

>From the mpd manual:

    bind_to_address <ip address or hostname or any>
           This specifies which address mpd binds to and listens on.
           Multiple bind_to_address parameters may be specified.  The
           default is "any", which binds to all available addresses.

           You can set a port that is different from the global port
           setting, e.g.  "localhost:6602".  IPv6 addresses must be
           enclosed in square brackets if you want to configure a
           port, e.g. "[::1]:6602".

           To bind to a Unix domain socket, specify an absolute path
           or a path starting with a tilde (~).  For a system-wide
           MPD, we suggest the path "/var/run/mpd/socket".

‘bind_to_address’ is the mpd config option corresponding to MPD_HOST.