Create a new mail/libmilter port distinct from mail/sendmail.

The idea here is that anyone wanting to configure a system where the
base is built WITHOUT_SENDMAIL in order to use an alternate MTA
doesn't then end up having to install sendmail somewhere in order to
use milters.

This also helps with the idea of making most shlibs in the base system
private.

Add new mail/libmilter port, content mostly copied or adapted from
mail/sendmail

Remove milter related parts of the mail/sendmail port. Note that
sendmail itself -- or any other MTA that can work with milters -- does
not need a LIB_DEPENDS on the new libmilter.

Update mail/sendmail/bsd.milter.mk so that milter ports will depend on
the new libmilter port. This is still conditional on the
WITH_SENDMAIL_PORT / WITH_SENDMAIL_BASE variables for backwards
compatibility.