split play.c and rename para_play to para_write

This separates the code to write an audio stream into several
independent pieces:

	write.h: definition of writer-related structures
	write.c: the para_write main program
	write_common.c: functions to be shared between para_write
		and para_audiod
	write_common.h: exported functions of write_common.c
	alsa_writer.c: writes the stream to an alsa sound device
	file_writer.c: writes the stream to a file

Other writers (aka output plugins) can be added easily.