gateways: change the approach for gateway generation
gateways: change the approach for gateway generation

Instead of injecting dynamic gateways after reading the config, we will
inject them before instead.  This gives us the opportinuity to check
for matches and remove disabled gateways, finally addressing #933 in a
sane way.

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core