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

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

Option depencies are (again) tracked automatically.

Hide the building and installation of libgssapi behind the
WITHOUT_KERBEROS knob. While GSS can be used for other things
some third party software (most notably ports/x11/kdelibs3)
takes the presence of libgssapi as an indication that kerberos
is available, and attempts to link with the kerberos libs. If
they are not available, the build will fail.

Because you might want to use GSS but not kerberos, add a knob
to re-enable it if WITHOUT_KERBEROS is present.

Document the new knob, and the new behavior of WITHOUT_KERBEROS.

Not objected and/or generally agreed to by: freebsd-arch

Problem discussed/analyzed in:
PR: ports/116484

Prepare to autogenerate the src.conf(5) manpage.