Is /etc/dhcpcd.d a good location for route-<interface> and rule-<interface> files?

Redhat derivatives use `/etc/sysconfig/network-scripts`.  Matching that would be consistent with this utility's goal of mimicking Redhat's mechanism for policy routing.

Perhaps more importantly, hook files such as `dhcpcd.exit-hook` are likely to be moved out of `/etc` into a directory that might very well be called `/etc/dhcpcd.d`.  Avoiding that future conflict might be a Good Idea.