rc.d/kdc: Support start of MIT krb5kdc

Some users wishing to use the MIT krb5kdc have discovered the
kdc script workaround applied to the MIT krb5 ports is insufficient.
Let's build into this rc script the smarts to determine whether
base or ports Hiemdal kdc is being invoked or the MIT krb5kdc.

While at it, remove kdc_start_precmd(). This will simplify a future
jail patch.

Suggested by: netchild
Original patch: netchild
Reviewed by: emaste, netchild
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43523