iwmbtfw(8): fix getopt flags, unify usage()/manual page contents

Seems like there was an intention to use command line switches
-m, -p and -v (each with argument), but there is no handling
for them.

Also, some enhancements to the usage()/manpage:

• use FreeBSD manpage style -- square brackets -- for denoting optional arguments in usage();
• show default directory path in usage();
• update manual: -f is the optional flag;
• show descriptions for -I/-D together: they are logically related.

Signed-off-by: Eygene Ryabinkin <rea@FreeBSD.org>
Reviewed by: wulf
MFC after: 1 month