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