Add rc.resume(8) alias for rc(8) to fix the manpage cross references
This issue was noticed when running `make manlint` as part of MFCing r342597 to
^/stable/11:
```
$ make -C share/man/man8 rc.8lint
mandoc -Tascii -Tlint rc.8
mandoc: rc.8:548:6: STYLE: referenced manual not found: Xr rc.resume 8
$
```
This is a followup commit to r339818.
MFC to: ^/stable/12