Gateway monitoring. Notify if back state.

[lexxai]

For now we have email messaging only if gw is down.

gw - Notification
MONITOR: WAN is down, omitting from routing group GWGROUP

But better will have option about Notify if GW back to normal state.

As I found it can be done here ```
/etc/inc/gwlb.inc

[athurdent]

2.4 beta sends those now.

MONITOR: GW_CBL is available now, adding to routing group ALLWAN
62.xxx.xxx.1|192.168.xxx.252|GW_CBL|12.661ms|4.793ms|13%|loss