etc/gss: Add MIT GSS glue

The MIT GSS uses different library names. And MIT KRB5 supports more
OIDs than our ancient Heimdal does.

The intention is to remove etc/gss when Heimdal is finally retired and
rename etc/gss-krb5 to etc/gss.

Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D50812