netifd: relax check in dhcp proto handler

Checking whether /sbin/udhcpc is a symbolic link breaks using the
DHCP proto handler inside procd-ujail where bind-mounts are used for
the resolved link. Check whether /sbin/udhcpc is executable instead
to allow using the proto handler for DHCP-provisioned containers.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/network/config/netifd/files/lib/netifd/proto/dhcp.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-[ -L /sbin/udhcpc ] || exit 0
+[ -x /sbin/udhcpc ] || exit 0
 
 . /lib/functions.sh
 . ../netifd-proto.sh