Instead of using a compile-time macro to suppress inclusion of the
menu entry to show UNIX command-line usage in the help menu, use a
sysconf setting instead.
Default is HIDEUSAGE=0, to include the entry for command-line usage.
Set HIDEUSAGE=1 to exclude that. Does not affect 'nethack --usage'
if player actually has access to the command-line.
Write up a description of how the command line works on UNIX and put
that in new file dat/usagehlp. Add support for
|nethack --usage | --help | -? | ?
to display it and exit.
Also add a menu entry for nethack's help command to show it during
play. That can be suppressed by uncommenting new '#define HIDE_USAGE'
in config.h since it won't be useful on servers that don't give
players access to command lines.
New genl_display_file() just writes to stdout. opt_usage(), which
calls it, might need some suid/sgid handling to make sure the output
is done as the player rather than as nethack.
doc/nethack.6 is already out of date again.
