GP: Fix the issue of a 0.0.0.0/0 "split"-include route by swapping the "split" route...

GP: Fix the issue of a 0.0.0.0/0 "split"-include route by swapping the "split" route with the default netmask.

GlobalProtect VPNs always or almost always send `<netmask>255.255.255.255</netmask>` (host route). If they
wish to include a true IPv4 default route (`0.0.0.0/0`), they send it a "split"-include route.

This interferes with NetworkManager users’ ability to use the "Use only for
resources on this connection" feature of NM's VPN plugins.  (Which basically
tells NM to use only split routes from the connection, and ignore a default route.)

This patch detects the case of a 0.0.0.0/0 IPv4 "split"-include route, and swaps it to become the default
default route.

See: https://gitlab.gnome.org/GNOME/NetworkManager-openconnect/-/merge_requests/12#note_818780

Signed-off-by: Daniel Lenski <dlenski@gmail.com>