MFC r271401:
r271401 (by asomers):

Conditionalize build of etcupdate(8) on MK_RCS. Since etcupdate calls
merge(1), which is part of the RCS package, it must not be installed if
WITHOUT_RCS update is set. Otherwise, it will produce confusing errors.

CR: https://reviews.freebsd.org/D691
Sponsored by: Spectra Logic

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

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

Revert r256095, r256120 (partial), r256121:

r256095:
- Add gnu/usr.bin/rcs back to the base system.

r256120:
- Add WITHOUT_RCS back to src.conf.5.

r256121:
- Remove UPDATING entry regarding gnu/usr.bin/rcs removal.

Requested by: many
Approved by: re (marius)
Discussed with: core

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