more slippery gloves

Move makeplural(body_part(FINGER)) into its own routine, with option
to substitute gloves when wearing such.

Wearing slippery gloves (ie, wearing gloves while having slippery
fingers) wouldn't let you put on a ring because you can't take the
gloves off, but removing a worn ring lacked the same restriction.

After changing that, teach prayer that slippery gloves is another
reason why a ring of levitation can't be removed.

src/write.c

@@ -1,4 +1,4 @@
-/* NetHack 3.6	write.c	$NHDT-Date: 1450261366 2015/12/16 10:22:46 $  $NHDT-Branch: NetHack-3.6.0 $:$NHDT-Revision: 1.17 $ */
+/* NetHack 3.6	write.c	$NHDT-Date: 1573346194 2019/11/10 00:36:34 $  $NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.20 $ */
 /* NetHack may be freely redistributed.  See license for details. */
 
 #include "hack.h"
@@ -109,7 +109,7 @@ register struct obj *pen;
         return 0;
     } else if (Glib) {
         pline("%s from your %s.", Tobjnam(pen, "slip"),
-              makeplural(body_part(FINGER)));
+              fingers_or_gloves(FALSE));
         dropx(pen);
         return 1;
     }