Make it harder for spambots to register

A re-implementation of
http://custom.simplemachines.org/mods/index.php?mod=1812 - most of the
code for Register.php has been copied verbatim from that mod. Currently,
a registration that fails due to this check returns an intentionally
unhelpful message telling potential users to e-mail us. No regular user
should ever see it, and I don't want the maintainer of some spambot see
a keyword-rich error message.