Balance fix to level drain and potions of restore ability

Right now, the punishment for being hit more than twice by a level
drainer pre-Quest is disproportionate; grinding back up to level
14 from level 13 takes a long time, and yet isn't particularly
difficult, just slow, and a potion of full healing will only
regain one of the lost levels (as only half the lost levels can
be regained this way).

Meanwhile, potions of restore ability are currently automatically
blanked by almost all spoiled players; they don't do anything that
doesn't have more convenient sources (unicorn horn or the spell),
so they're only useful in the very early game for getting poison
resistance.

This commit aims to fix both problems, by allowing potions of
restore ability to restore lost experience levels, in addition to
lost attribute points; an uncursed potion restores one lost level
(with multiple potions making it possible to hit the cap), a
blessed potion restores all of them. That gives players an
incentive to keep them around rather than blanking them. (Notably,
the spell and tool were not changed the same way; for restoring
levels, you need to use the potion.)

src/exper.c

@@ -293,9 +293,11 @@ boolean incr; /* true iff via incremental experience growth */
             u.uexp = newuexp(u.ulevel);
         }
         ++u.ulevel;
+        pline("Welcome %sto experience level %d.",
+              u.ulevelmax < u.ulevel ? "" : "back ",
+              u.ulevel);
         if (u.ulevelmax < u.ulevel)
             u.ulevelmax = u.ulevel;
-        pline("Welcome to experience level %d.", u.ulevel);
         adjabil(u.ulevel - 1, u.ulevel); /* give new intrinsics */
         reset_rndmonst(NON_PM);          /* new monster selection */
     }