MFC r303784, r303785, r305620:

r303784:
etcupdate: directly use diff3(1) instead of merge(1)

During the last attempt to rmeove GNU rcs, 2 blockers were spotted:
We need an ident(1) and etcupdate(8) uses merge(1).

Now nothing should prevent to remove rcs from base

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D7401

r303785:
always install etcupdate

Now that etcupdate does not depend on rcs anymore there is no need to
conditionnally install it

r305620: (by vangyzen )
etcupdate: preserve the metadata of the destination file

When using diff3 to perform a three-way merge, etcupdate lost the destination
file's metadata. The metadata from the temporary file were used instead.
This was unpleasant for rc.d scripts, which require execute permission.
Use "cat >" to overwrite the destination file's contents while preserving its
metadata.

Reviewed by: bapt
Sponsored by: Dell Technologies
Differential Revision: https://reviews.freebsd.org/D7817