Added support for import/export of shared avatar bin files (libraries of

avatars).
The library (from where the user picks stock/shared avatars) is
text/avatars/*.bin.
Network-shared avatars .bin files (meeting the requirements) are imported
as <qwk-id>.<filename.bin> in this directory.
The sysop chooses which locally created avatar libraries to share
with the "export=<sub> -f=<filename>".
There's no de-duplication at this time.
Move some functions from avatar_lib.js to avatars.js.