portable_io.h: Provide big-endian versions and use them for aac.

portable_io.h: Provide big-endian versions and use them for aac.

The aac audio format handler code contains some instances that read a
big-endian encoded 32 or 64 bit number from a buffer. While for the
32 bit case there is a helper function aac_read_int32(), the 64 bit
case is open-coded.

We already have similar functions for the conversion of little-endian
entities. This patch adds their big endian counterparts as inline
functions to portable_io.h and changes the callers to use those.

The patch also gets rid of two fprintf() statements in write_portable()
which were commented out for ages.