Support additional arguments in AreaFix request message subject, beyond pass

Apparently AreaFix and other area managers assume password never have spaces
in them, so they can support additional arguments following the password in
the AreaFix request.

This change also now disallows setting AreaManager passwords with spaces in
them.

I don't yet support any of the additional AreaFix request options (e.g. -L,
-R, -Q), but am considering it.

A bunch of sprintf() -> snprintf() conversions are included in this commit
(better string buffer security, probably eliminate a Coverity issue or two).