Right now, the only preference is reverse mail listings (oldest first
or newest first). These settings are only used when reading "your mail",
not any other kind of mail reading.

bbs.read_mail() now returns the user-adjusted loadmail_mode value and
this allows us to determine the user's preferences and save them after
this function/method is called. A readmail_mod can now return a number
(other than 0) and that will be used as the return value of this method.

sbbs_t::readmail() now does the adjustment of the passed lm_mode before
calling any installed readmail_mod, so if for example, deleted message
viewing is enabled by the sysop, those LM_* flags might be set now in
the argument to the readmail_mod, wherase they never would before.

There is not yet any way for the sysop to set a new user's default
mail_settings, they'll just default to 0 for now.

email_sec.js will get some adjustments to use/store the
user.mail_settings next.