pcm_get_htimestamp can now use CLOCK_MONOTONIC

Previously the timebase was not documented.  Now a new pcm_open flag
can be used to specify the clock.  If flag PCM_MONOTONIC is specified,
then pcm_open uses SNDRV_PCM_IOCTL_TTSTAMP with parameter
SNDRV_PCM_TSTAMP_TYPE_MONOTONIC to request timestamps in CLOCK_MONOTONIC.

Change-Id: I40ce359cb38d686cbb2521fb0602a8a17ab4f925