openssh: clarify krb5 use in freebsd-configure

freebsd-configure.sh runs configure twice, --with-kerberos5 and
--without-kerberos5, in order to build a config.h that defaults to
kerberos5 disabled, and a small config file that represents the
differences.

Rename config.h.orig to config.h.kerberos5 to clarify the intent of this
script.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 5e4dd21fd60958ceef4e4fd14b000844b957287e)