#28 Restrict user emails to a certain domain

[ninjawarrior13]

Allow the admin to control users who register, such that
only users who have an email ending in @domain.com
(for example) can register.

This would be extremely useful in organizations where
everyone who has a valid e-mail address ending in
@domain.com (again, for example) also has access to
newgroups that are supposed to be login-only accesable.

This way, the users getting access would be users who
have access to the login-only newsgroups.

This way, the newgroups don't have to be improperly
exposed to anonymous users or users without permission.

Ideally, subdomain support could work too (for example,
@sub.domain.com, @sub2.domain.com), etc, and
multiple domains as well.

The code basically should check to make sure whatever
comes after the @ sign in the registration form is equal
to a certain entry or entries in the MySQL database.