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