ir_nec: optionally accept input signals that include the carrier

Introduce optional detection of a carrier signal. Immediately "go active"
when edges are seen. "Go inactive" again in the absence of edges in a
specified period of time. Cope with input signals that already had the
carrier removed.

By default carrier detection is disabled, to remain pixel compatible to
the previous implementation. When a carrier frequency is specified and
thus detection is enabled, edges of already filtered input are shifted
by one carrier period, and thus changes the output of the decoder. For
unfiltered inputs that still contain the carrier, detection of activity
is reliable and immediate, but the active phase is extended by one
period of the carrier frequency (which is considered acceptable).