Handle tar type flag 'K' (GNU long link extension)

Handle tar type flag 'K' (GNU long link extension)

Handle tar type flag'K' (GNU long link extension)
Also report unexpected filename in headers with long name extension type
Also clean up some cruft in archive_tar.cc

Currently, the only existing package which contains type flag 'K'
entries is f21-backgrounds-extras.

This also now correctly handles tar archive header linknames of exactly
100 characters (even when not preceeded by a 'K' type header), where the
linkname field is not null terminated as it should be, rather than
allowing the 'ustar\0' magic immediately following it to be interpreted
as part of the linkname.