Rewrite all the packing stuff so that it converts 'f' strings to CP850

Rewrite all the packing stuff so that it converts 'f' strings to CP850
on encoding.  (Tough luck if your Samba codepage is not 850.)  It's
much cleaner and possibly more efficient now too: just build one big
list of strings, then zip it up at the end.

Still need to do this for unpacking.
(This used to be commit 2480d385db0527b71cc5cd23c4fab4655df84485)