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.

NEWS

@@ -79,6 +79,17 @@ CHANGES WITH 262:
         * bootctl, systemd-oomd, systemd-repart, systemd-sysusers, and
           systemd-tmpfiles now accept -n as a short option for --dry-run.
 
+        Changes in systemd-tmpfiles:
+
+        * systemd-tmpfiles now rejects non-empty argument fields for tmpfiles.d
+          line types that do not use the argument field. This is a compatibility
+          change: fields that were previously ignored after a warning will now
+          cause the tmpfiles.d configuration to be rejected, so downstreams
+          should audit shipped tmpfiles.d snippets before upgrading.
+
+        * The r and R tmpfiles.d line types now honor the age field when
+          systemd-tmpfiles is invoked with --clean.
+
         Changes in the TPM Subsystem:
 
         * The way that NvPCRs are anchored has been improved. They are now