Fix panic when Dragon Caves are not generated

Without this fix, dname_to_dnum() would panic at the start of the game
if the Dragon Caves were not generated in that game's dungeon layout.
Now dragon_dnum is set to n_dgns to indicate that the branch is not
present.

Fixes #66 on GitHub.
Don't flip Sokoban levels

Flipping doesn't make the puzzles any more interesting, and makes the
game less appealing to NetHack players that hate Sokoban and rely on
spoilers to bypass or tolerate it.
Put bool/enum option desc on own line in dialogs

Descriptions for boolean and enum options now show up on their own line
instead of crowding up the menu title when changing them.  Ideally, I'd
do this for int and string options too, but their dialogs don't yet
support anything like this.

Closes #63 on GitHub.
Show wield in inv action menu of chained ball

This fix is especially welcome for convicts, who start with a heavy iron
ball chained to them and are suited to wielding it.

Fixes #25 on GitHub.
Show wear in inv action menu of wielded armor

This also includes armor in the alternate weapon slot, is quivered or
happens to be a carried or invoked artifact (if the last two are ever
added to the game).  These are all "worn" slots that the game still
allows wearing from.

Fixes #26 on GitHub.
Hide wield from inventory action menu of armor

From experience, it was very easy to typo wield instead of wear in the
inventory action menu of armor, so this removes it.  If someone really
wants to wield armor, they can still use the 'w' key.
Use '0' for boulders in ASCII display mode

This was already the case for Unicode graphics, but many NAO players
that use ASCII still set the boulder option to '0'.

This is done the way it is because doing it any further up affects game
behavior, special level descriptions, etc.

Closes #68 on GitHub.
Autoexplore:Don't stop for fountains/sinks/thrones

They usually aren't worth visiting when they're first encountered.

Closes #67 on GitHub.