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
    (cherry picked from commit 4985aa8c6e89451996e659a39fec7646e9e25f76)
    
    Cherry-picked from: 4985aa8c6e89451996e659a39fec7646e9e25f76
    Resolves: #1807395
    
    patch_name: 0069.patch
    present_in_specfile: true
    location_in_specfile: 69
    squash_commits: true