dungeon open failure

dungeon open failure

We've been getting numerous complaints from people
about "dungeon failure", often related to attempts
to start NetHack from within various zip utilities
that present a folder-like view.

The dungeon failure was actually misleading. The
real problem was a dlb file open failure, but the
return value of dlb_init() was not being checked
in pcmain.

This moves the dlb_init earlier in the startup,
checks for failure, and provides some feedback
around the common zip utility problem for win32.