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.