bgscan: Add signal strength change events

bgscan: Add signal strength change events

This allows bgscan modules to use more information to decide on when
to perform background scans. bgscan_simple can now change between
short and long background scan intervals based on signal strength
and in addition, it can trigger immediate scans when the signal
strength is detected to be dropping.

bgscan_simple takes following parameters now:
short interval:signal strength threshold:long interval
For example:
	bgscan="simple:30:-45:300"