String array for default rate plugin list

String array for default rate plugin list

Change the rate converter type to allow string arrays in addition to
a string.  When a string array is given, the rate plugin probes each
string and try to load the converter plugin in the order of the list.

For example, you can set
	defaults.pcm.rate_converter	[ "samplerate" "linear" ]
so that samplerate plugin is preferred to linear plugin if it's
installed.