Rework the logic for installing the pkg(8) configuration.

'quarterly' package sets do not exist for head, so explicitly
install the 'latest' configuration file there. Otherwise,
fall back to the original conditional evaluation to determine
if the 'latest' or 'quarterly' configuration file should be
installed.

Reported by: manu
Reviewed by: manu
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC (Netgate)