Various minor changes and fixes.

Various minor changes and fixes.

- Automatic newline when calling DEBUG() or ERROR()
- Use ERROR_WITH_ERRNO instead of "%m" conversion specifier
- Security data cannot be NULL anymore; just allocate one with zero entries.
- Cleaned up failure case code in some functions, also documented whether the
  WIMs are valid after wimlib_add_image(), wimlib_delete_image(), and
  wimlib_export_image() fails or not.