Try to unify "back on solid ground" messaging

Put everything through a single function that can handle all the
complicated parts of using the correct proposition for different terrain
types, and will not just call things "solid ground" indiscriminately.
This got complicated but I'm not sure if it's possible to do it much
simpler while still using the distinct names for each type of terrain
(unless you are OK with the sentences sounding sort of wonky).

src/hack.c

@@ -2826,11 +2826,7 @@ pooleffects(
             } else if (is_lava(u.ux, u.uy)) {
                 You("leave the %s...", hliquid("water")); /* oops! */
             } else {
-                char icebuf[BUFSZ];
-                You("are %s %s again.",
-                    (Levitation || Flying) ? "over" : "on",
-                    is_ice(u.ux, u.uy) ? ice_descr(u.ux, u.uy, icebuf)
-                      : "solid land");
+                back_on_ground(FALSE);
                 iflags.last_msg = PLNMSG_BACK_ON_GROUND;
             }
         } else if (Is_waterlevel(&u.uz)) {