[Exim] FreeBSD port updated to exim-4.24

Hi folks,

The FreeBSD port for Exim has been updated to exim-4.24, thanks to work
done in advance by Oliver.

Folks who used mysql, ldap or postgresql lookups and manually overrided
the dependency versions for those packages in the past will need to read
the commit message closely.

Also, folks who use configure files in places other than
$PREFIX/etx/exim will need to check out the commit message, because
ALT_CONFIG_PREFIX has been enabled by default.

Other than that, should be a solid upgrade, especially if you use
portupgrade.

Note that FreeBSD 4.9-RELEASE goes out with exim-4.22 (PORTREVISION 4).
That'll teach Tom to take a holiday. ;-)

Ciao,
Sheldon.