(main): New options --human-readable (-h) and --megabytes (-m).

(main): New options --human-readable (-h) and --megabytes (-m).
(human_readable_1k_blocks): New function.
(print_header): Honor new options.
(show_dev): Honor new options.
Add ifdef'd-out code to avoid printing lines for mount points that
match ^/auto/.* and ^/tmp_mnt.*.  Rather than hard-coding those
two prefixes, the code should allow the user to specify a regular
expression.  From Larry McVoy (lm@sgi.com).

(human_readable_1k_blocks): Likewise
(usage): Update usage message to reflect additions.