net-online: only process symbolic links in /sys/class/net

The /sys/class/net directory contains files which are not symlinks if
the system has bonded devices [1].  We should ignore these files.

This fixes #196.

[1] https://elkano.org/blog/manage-interface-bondings-sysfs-interface/

init.d/net-online.in

@@ -23,6 +23,7 @@ get_interfaces()
 {
 	local ifname iftype
 	for ifname in /sys/class/net/*; do
+		[ -h "${ifname}" ] && continue
 		read iftype < ${ifname}/type
 		[ "$iftype" = "1" ] && printf "%s " ${ifname##*/}
 	done