pulse_worker: implement "single pulse" mode to work around very overloaded sound file sources.

In this mode, any series of overlapping pulses will be ignored after its first
pulse.
source, sources/*: add source-global option "single" to enable single pulse mode for per-source worker.

Per-channel (global) workers still won't take this into account. This will need
another command-line parameter and a way to pass options to the core.