Fix #339 - added configurable grace period per adapter.
service: added s_is_enabled() function for checking enable chain
Issue #1532 - service: ensure that only fully enabled services are used.

This uses the new s_is_enabled() callback routine to check if the
service chain is completely enabled. For DVB services this will
also check the container mux and adapter.
Fix #1542 - timeshift: stop crash if speed/skip request w/o service

This can happen because XBMC may send requests even though the sub
has not yet started and the relevant links been made.