add an "AUTOQUIT" flag to mix and sub aprocs. If the flag is

add an "AUTOQUIT" flag to mix and sub aprocs. If the flag is
set, the mix aproc will exit once there are no more input
streams, similarly the sub aproc will exit once there are no
more ouput streams. If the flag is not set, the mix aproc will
generate silence, and the sub aproc will drop samples. By
default this flag is set, so no behaviour change.

ok jakemsr