ALSA: rewrite capture plugin

 * use a thread instead of short interval polling,
 * do not assume latency is shorter than a period,
 * negotiate sample format instead automatically,
 * map all known ALSA formats to VLC codecs,
 * negotiate channels and sample rate,
 * partial support for 3-6 channels (missing preferences),
 * negotiate buffer and period times according to caching,
 * tolerate off-by-one period size due to rounding,
 * use the "default" device by default rather than "hw",
 * remove old-style hardware-only suggestions in debug messages.