switch default krb5 - except on (Net|Free)BSD

[nia <nia%NetBSD.org@localhost>]

Hi all,

heimdal appears to have some portability problems on
various more obscure platforms, and is overall the
less popular kerberos implementation.

Only NetBSD and FreeBSD ship heimdal as the default,
and FreeBSD is considering switching to MIT kerberos.

following a successful change in dreckly, I'd like to
flip the default kerberos to MIT except on NetBSD
and FreeBSD.