Add tne necessary Makefiles and header files to facilitate building
MIT KRB5 as part of buildworld. Nothing will build until the
WITH_MITKRB5/MK_MITKRB5 option has been plumbed in Makefile.inc1.
Before any changes to Makefile.inc1 are made to enable MIT KRB5,
additional commits to other affected software will need to be committed.
krb5/Makefile was inspired by kerberos5/Makefile. The Makefiles in
krb5/util and krb5/lib were inspired by those in lib/libc and in
lib/ncurses.
Differential revision: https://reviews.freebsd.org/D50695
Sponsored by: The FreeBSD Foundation
Lexi Winter