A new global option enables sending this vendor ID to prevent Cisco
devices from narrowing the initiator's local traffic selector to the
requested virtual IP, so e.g. 0.0.0.0/0 can be used instead.
This has been tested with a "tunnel mode ipsec ipv4" Cisco template but
should also work for GRE encapsulation.
Closesstrongswan/strongswan#180.
