We'll try setting an initial value of 50ms, and halve it each
time snd_pcm_hw_params fails with -EPIPE.

This way we'll can use a larger (50ms) period_size whenever a device
supports it, and automatically pick smaller ones if we can't set
larger ones.

This removes the calculation borrowed from libao (svn) as well.

Other minor things:
"Alsa" => "ALSA" in error messages
_US appended to *_TIME constants so we won't get confused
(shank's request)

git-svn-id: https://svn.musicpd.org/mpd/trunk@4438 09075e82-0dd4-0310-85a5-a0d7c8717e4f