Initial support for email "tags"

Incoming email is now scaned for To: headers in the format <name>#<tag>@host

Tag is extracted, and if it is not found in data/user/%04d.smtptags it is
sent to the user.  If it DOES match a line in the smtptags file, the email
is rejected with a no such user error.

Using this allows you to easily create "temporary" email addresses.  Once you
start recieving spam on one tag, just disable it.

TODO: Make the separator characters configurable.