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.

Reviewed by: eugen
Approved by: jtl (mentor)
MFC after: 1 week
MFC to: ^/stable/12
Differential Revision: https://reviews.freebsd.org/D19158