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/myfprintf.c

@@ -17,7 +17,7 @@
  */
 
 #include "myfprintf.h"
-#include "interface.h"
+#include "client.h"
 #include "path.h"
 #include "utils.h"
 #include "os_compat.h"