While here, if we cannot find ypbind to work out how the hook script
should be configured, guess according to OS.

This solves the warnings on all OS other than Linux who don't have
a supporting resolvconf for deprecating DNS because they never
emit the NOCARRIER_ROAMING reason.

This is given when the OS supports the concept of wireless roaming
or the IP setup can be persisted when the carrier drops.

When this happens, routes are moved to a higher metric (if supported)
to support non preferred but non roaming routes.
The `interface_order` hook variable will now order the interfaces
according to priority and move roaming interfaces to the back of the
list.
If resolvconf is present then it is called with the -C option
to deprecate DNS and if carrier comes back it is called again with the
-c option to activate it once more.

As part of this change, default route metrics have been changed to
support a larger number of interfaces.
base metric		1000		(was 200)
wireless offset		2000		(was 100)
IPv4LL offset		1000000		(was 10000)
roaming offset		2000000

With this set dhcpcd will not configure anything on the host.
The expectation is that a 3rd party script will instead.