nasm.c: tidy up the help text and break it into topics

The help output has gotten way too long to be shown on a single
command line. It can of course be piped to a pager, but to be a little
nicer to the user, break it up into subtopics that can be individually
displayed. --help all (-h all) can still show all the help information
as a single data dump.

Signed-off-by: H. Peter Anvin <[email protected]>

Author: H. Peter Anvin