nethack.sh: test -e vs test -f

nethack.sh: test -e vs test -f

Change the test for whether fonts.dir exists (added to the script
in 3.6.0, for automatically setting up possible use of the NH10 font
under X11) from 'test -e file' to 'test -f file' since the latter
seems to be more universally available.  When present, fonts.dir is
plain text, so a test for "exists and is a regular file" rather than
one for general existance is appropriate.