tmpfiles: reject extra argument fields and make 'r'/'R' support age field (#43195)

This updates systemd-tmpfiles in two small areas:

- Reject non-empty argument fields for tmpfiles.d line types that do not
consume the argument field, instead of warning and silently ignoring
them.
- Let `r` and `R` tmpfiles.d entries honor the `Age` field when
`systemd-tmpfiles --clean` is used. The existing `--remove` behavior
remains unconditional.

The completed TODO entries are removed, and NEWS/man page documentation
is updated for the visible behavior changes.

diff --cc NEWS
Simple merge