Enable kernel reproducible builds on stable branch

When the stable/13 branch was created the userland knob (in
share/mk/src.opts.mk) was changed to default to yes, but the kernel
default was not changed. These two should always have the same default.
Regenerate src.conf.5, as the differing defaults previously confused
makeman.

Fixes: bfd15705156b ("Create the stable/13 branch")
Sponsored by: The FreeBSD Foundation
Approved by: re (cperciva)

(cherry picked from commit d8644a5d2458588dd7d8e44bd23c6cacaead19b7)
(cherry picked from commit 3a8e15354ec58abeec2ee2fe96cbabb46083d183)