Allow in-place overwrites when unmounting read-write mounted WIM

Allow in-place overwrites when unmounting read-write mounted WIM

- Add WIMLIB_UNMOUNT_FLAG_REBUILD and WIMLIB_UNMOUNT_FLAG_RECOMPRESS
- Make lock_wim() set a flag WIMStruct.wim_locked to prevent double-locks
- Maintain the inode list for each image through a read-write mount (not
  strictly necessary yet, but makes sense to keep it correct)
- overwrite_wim_inplace():  Search for new streams by searching directly through
  the lookup table for entries that are not located in the WIM being
  overwritten.