server_socket: use resolve_host_port() instead of getaddrinfo()

Allow port specification in "bind_to_address" settings.

doc/mpd.conf.5

@@ -83,6 +83,10 @@ 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.  For a
 system-wide MPD, we suggest the path "\fB/var/run/mpd/socket\fP".
 .TP