Alsa: revise latency use and calculations, since the latency should be equivalent to one Alsa period less than the Alsa ringbuffer (was based on the whole).  Also fixes failure if zero latency was specified.

git-svn-id: http://subversion.assembla.com/svn/portaudio/portaudio/trunk@1893 0f58301d-fd10-0410-b4af-bbb618454e57
OSS: revise default latency reporting: previous fixed values could not actually be achieved with OSS power-of-2 sizes.  Instead try a configuration and correctly report the default latency based on one less than the number of fragments; also fix the stream component 'bufSz' calculation.

git-svn-id: http://subversion.assembla.com/svn/portaudio/portaudio/trunk@1894 0f58301d-fd10-0410-b4af-bbb618454e57