Support for joining WIMs.

Support for joining WIMs.

- join command for imagex
- wimlib_join() function
- wimlib_get_part_number() functions
- Various modifications in other files to make this possible.  wimlib_open_wim()
  will open a split wim given the WIMLIB_OPEN_FLAG_SPLIT_OK flag.
- Allow finish_write() to skip writing lookup table in case it is written
  elsewhere (join_wims())
- Show more header flags in wimlib_print_header() (including
  WIMLIB_HDR_FLAG_SPANNED)
- Allow images with no name, and when extracting them, extract to directory
  named after image number rather than name