Lots of changes

- Add progress functions
- Rename wimlib_mount() and wimlib_unmount() to wimlib_mount_image() and
  wimlib_unmount_image()
- Remove wimlib_add_image_from_ntfs_volume() and
  wimlib_apply_image_to_ntfs_volume() in favor of flags to wimlib_add_image()
  and wimlib_extract_image()
- Add progress callback functions, and make them work for NTFS apply as well as
  normal apply
- Consolidate some of the extraction code into extract_single_image().
- Make serial WIM reading work when applying to NTFS as well as in the normal
  extraction mode.
- Update documentation in wimlib.h.