Replace any undefined tag values with empty string for ID3 tagging

Avoids rare Win64 problems with junk data in TRCK and TPOS frames
Added category field to metadata

Set to first category (in taxonomic order).  Available as a substitution
parameter.  Used to set genre tag.
Added firstbcastdate and lastbcastdate fields to metadata

Date parts of firstbcast and lastbcast fields.  Also available as
substitution parameters.
Force --fatfilename with --whitespace (Windows only)

If --whitespace is used, non-alphanumeric characters are allowed in
output file name, so --fatfilename is necessary to filter them for NTFS.
Enforce 256-char path limit on Windows only