keepalived: add new uci section config interface_up_down_delays

keepalived: add new uci section config interface_up_down_delays

If an interface that is being used (or tracked) by a VRRP instance goes to
down state, the VRRP instance(s) will, by default, immediately transition to
FAULT state, and when all relevant interfaces are back up again the VRRP
instance(s) will immediately transition to BACKUP state.

This can cause problems if interfaces are bouncing, and so delays can be
specified between the interface state change and the transition to
FAULT/BACKUP state. If the interface returns to its original state before
the delay expires, no associated VRRP instance state transition will occur.

New uci section 'interface_up_down_delay':

config interface_up_down_delays
	option device <device>
	option down_delay <number in seconds>
	option up_delay <number in seconds>

Signed-off-by: Florian Eckert <fe@dev.tdt.de>