Clean up and improve snd_pcm_update_hw_ptr*() functions.

snd_pcm_update_hw_ptr() tries to detect the unexpected hwptr jumps
more strictly to avoid the position mess-up, which often results in
the bad quality I/O with pulseaudio.

The hw-ptr skip error messages are printed when xrun proc is set to
non-zero.

Signed-off-by: Takashi Iwai <tiwai@suse.de>