network-legacy/ifup: fix ip=dhcp,dhcp6 setup_net logic

network-legacy/ifup: fix ip=dhcp,dhcp6 setup_net logic

Previously, we were doing `setup_net` from `ifup` for any setup that
wasn't DHCP, since those are already taken care of by `dhclient-script`.

The issue is that the case-statement we use to detect this doesn't catch
options like `ip=dhcp,dhcp6`.

Fix this by reworking the logic here to just check if a
`setup_net_$netif.sh` hook exists. If so, then we know that `setup_net`
will be called for this interface later.

This was causing issues in RHCOS which now ships with `ip=dhcp,dhcp6` to
support IPv6 environments[1]. The code here would make us do `setup_net`
pre-emptively which IIUC would then cause the initqueue to finish
earlier even if we had more udev netif events to process.

[1] https://github.com/coreos/coreos-assembler/pull/1067

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1803926