write: various cleanups

write: various cleanups

- When writing or overwriting WIM file, fill in a new variable
  wim->out_hdr instead of overwriting wim->hdr which was subject to bugs
- When writing WIM file, create new header from scratch instead of
  copying old one
- Remove redundant private flags
- Remove debug statements (they are rarely used, outdated, and now
  missing from most parts of the library)