new achievement: drawbridge tune

Use up the last available bit for achievements:
"You learned the tune to open and close the castle's drawbridge."
(More can still be added but xlogfile will need another field to
track a second set of 31 in order to keep its achievement bitmask(s)
within portable size.)

As achievements go, it's not very exciting, but players who normally
destroy the drawbridge have to choose whether to earn an achievement
first since once it's gone, there's no way to find out the tune
(either via prayer reward or successful Mastermind).  I'm guessing
that most will probably decide to ignore this achievement since it
has no effect on the outcome of the game.  However, that might not
be true for future tournament play.

There's no need to bump EDITLEVEL for this; room for recording one
additional achievement is already allocated.

include/you.h

@@ -98,7 +98,7 @@ enum achivements {
        so that disclosing them can use the gender which applied at the time */
     ACH_RNK1 = 23, ACH_RNK2 = 24, ACH_RNK3 = 25, ACH_RNK4 = 26,
     ACH_RNK5 = 27, ACH_RNK6 = 28, ACH_RNK7 = 29, ACH_RNK8 = 30,
-    /* foo=31, 1 available potential achievement; #32 currently off-limits */
+    ACH_TUNE = 31, /* discovered the castle drawbridge's open/close tune */
     N_ACH = 32     /* allocate room for 31 plus a slot for 0 terminator */
 };
     /*
@@ -117,7 +117,6 @@ enum achivements {
      *  entered Fort Ludios level/branch (not guaranteed to be achieveable),
      *  entered Medusa level,
      *  entered castle level,
-     *  opened castle drawbridge,
      *  obtained castle wand (handle similarly to mines and sokoban prizes),
      *  passed Valley level (entered-Gehennom already covers Valley itself),
      *  [assorted demon lairs?],