aplay: Refactor playback code

aplay: Refactor playback code

This introduces read_header function which tries
to read the header of an audio file in order to determine
its type.

This has the following effects:
	(1) makes code easier to read
	(2) don't abort if file size is less than expected header

(2), allows us to play small files with size smaller than any
supported audio file headers.

Suggested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>