NX Spec: Include 82-byte header in audio; rename MP3 to audio.

This is not a breaking change - only consumers of the audio data need to
slightly tweak their parsing. But otherwise it remains fully backward-
compatible, so this will be brought forward from PKG5.

Signed-off-by: angelsl <hidingfromhidden@gmail.com>
NX Spec: Squash PKG5 commits.

NX Spec: Bring PKG5 up to master branch.
NX Spec: Use <code> instead of <span ...>
NX Spec: Update acknowledgements; @aaronweiss74
NX Spec: Metadata fields in byte arrays and header.
NX Spec: Rewrite PKG5. Now backwards-compatible with PKG4 for servers.
NX Spec: Correct inconsistency.
NX Spec: Add string null terminator, and Metadata field for WZ-specific data.
NX Spec: Allow omission of bitmaps/audio. (WZ)
NX Spec: Fix small errors; more specific.
NX Spec: Missed one MP3.
NX Spec: Refine WZ-specific data types.
NX Spec: Add WZ-specific data types.
NX Spec: Begin 4th version.

Signed-off-by: angelsl <hidingfromhidden@gmail.com>
NX Spec: Missed one table header. Make bitmap definition more concise.

Signed-off-by: angelsl <hidingfromhidden@gmail.com>