gssd: Add support for MIT KRB5

[cy]

Summary

Fix gssd build when building with MIT KRB5.

krb5_get_init_creds_opt_set_default_flags() is not an MIT KRB5 function.
It is also not needed to resolve the default keytable.

Sponsored by: The FreeBSD Foundation

Test Plan

Running here for a month.

[rmacklem]

usr.sbin/gssd/gssd.c

1196

Very minor nit (which means don't do it unless
you agree). Maybe use HEIMDAL or a comment
that indicates this is a HEIMDAL call instead of
"not MIT"?

[cy]

usr.sbin/gssd/gssd.c

1196

Good idea. I will add a comment, as I use the define MK_MITKRB5 elsewhere. That keeps every source #ifdef in the tree consistent.

I've added the comment for the other files with this same #ifdef, pam_krb5 and pam_ksu.