The syntax I gave in the --help output for --enable-masquerade incorrectly

implied that the earlier options were prerequisites for the later options
(pointed out by Scott Blachowicz).  No compact, non-confusing way to specify the
real syntax that I can see, so I'll just remove the []s and leave the explaining
about it being okay to specify a subset to the INSTALL file.

Also added a TODO item to put in proper parsing of the masquerade: line and/or
--enable-masquerade so if you misspell an option you find out about it, rather
than just having it silently fail (this weakness was currently only documented
in comments in the mts.conf-handling code).