renamed interface.c to client.c

I don't believe "interface" is a good name for something like
"connection by a client to MPD", let's call it "client".  This is the
first patch in the series which changes the name, beginning with the
file name.

src/main.c

@@ -16,7 +16,7 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
-#include "interface.h"
+#include "client.h"
 #include "command.h"
 #include "playlist.h"
 #include "directory.h"