[Dovecot] patch: mailboxcasecmp()

Here is a patch which adds mailboxcmp() and mailboxcasecmp() functions, similar to mailbox_equals(). Names were chosen to match strcmp() and strcasecmp(). I needed this for Johaness Berg's dspam plugin. It watches a folder "SPAM" and forcing this to be uppercase is unacceptable for me. (I also had to modify the plugin to use the new routine.)

It's against dovecot-1.0.beta8 but should be trivial to port to CVS.

-frank