sw_limits: start msec timeout period only after start() call

sw_limits: start msec timeout period only after start() call

When application code used the common SW limits API, the call sequence
of init() then set() then check() already kept expiring, which is rather
unexpected. The timeout period should only start when start() is called,
check() should not signal expiration before the start() call.

The specific use case is the combination of an msecs timeout and capture
ratio when triggers are used. The post-trigger period only starts when
the trigger match was seen, even though its length is already known when
the acquisition starts. It's desirable to run the start() call for the
post-trigger timeout late, and not terminate the acquisition before the
trigger match.