Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Fix setting of MK_GSSAPI option by bsd.own.mk; its value should
default to the value of MK_KERBEROS unless set explicitly by
WITH_GSSAPI/WITHOUT_GSSAPI. (This introduces another type of
MK_* variables which itself is questionable.)

- Teach tools/build/options/makeman script that generates the
src.conf(5) manpage about the new type of MK_* variables.

- Fix broken logic in lib/Makefile.