format_pcm: Track actual header size of .au files

Sun's Au file format has a minimum data offset 24 bytes, but this
offset is encoded in each .au file. Instead of assuming the minimum,
read the actual value and store it for later use.

ASTERISK-20984 #close
Reported by: Roman S.
Patches:
	asterisk-1.8.20.0-au-clicks-2.diff (license #6474) patch
	uploaded by Roman S.

Change-Id: I524022fb19ff2fd5af2cc2d669d27a780ab2057c