main/openrc: make netmount init.d work with Busybox umount
Busybox's umount does not support the "-O" option so only run "umount -a -O _netdev" if util-linux's umount is installed, otherwise loop through /proc/self/mountinfo instead and for each entry that contains "_netdev" in its option list then use Busybox's umount on it.
Fixes #9923 (closed) and #13789 (closed).