Create the tun device after dropping root privileges.

Currently, clatd creates the clat4 interface as root using
root's dac_override abilities. Instead, change the drop_root code
to acquire membership in the AID_VPN group, and use that
membership to create the interface. This removes the need for
dac_override.

Change-Id: I1f824254f52a441f21c5b7963d9993be88cea2db
1 file changed