openfortivpn: Use netifd for script rather than init script

openfortivpn: Use netifd for script rather than init script

By using the netifd for open fortivpn we are able to set up multiple
VPN connections and manage them through the netifd toolset.

This also adds support for binding an openfortivpn client to a given
interface, in which case when that interface comes online, the vpn
will be initiated via a hotplug script.

This is a breaking commit and configurations will need to be migrated
from openfortivpn.config into the /etc/config/networks.

Example configuration via /etc/config/network:

config interface 'ftvpn'
        option proto 'openfortivpn'
        option server 'example.com'
        option username 'USERNAME'
        option password 'PASSWORD'
        # optional arguments follow
        option local_ip '192.0.5.1'
        option port '443'
        option iface_name 'wan'
        option trusted_cert 'CERT_HASH'
        option set_dns '0'
        option pppd_use_peerdns '0'
        option metric '10'

Signed-off-by: Aaron Goodman <aaronjg@stanford.edu>