alsaloop: added option prateshift for PLAYSHIFT ctl elem used in PLAYSHIFT

alsaloop: added option prateshift for PLAYSHIFT ctl elem used in PLAYSHIFT

If snd-aloop device is on playback side, the required sync mode is
PLAYSHIFT. That means Loopback ctl elem "PCM Rate Shift 100000" of the
corresponding capture side of the Loopback pipe must be controlled (by
a reciprocal).

ASCII name of the playback rate shift ctl elem is specified with newly
added option -x/--prateshift, e.g.:
  -P hw:Loopback,0 -S playshift \
  -x iface=PCM,name='PCM Rate Shift 100000',device=1

Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>