Extensions to rmdir and mkdir utilities

Extensions to rmdir and mkdir utilities

* Rework GIT_DIRREMOVAL values to GIT_RMDIR flags, allowing
  combinations of flags
* Add GIT_RMDIR_EMPTY_PARENTS flag to remove parent dirs that
  are left empty after removal
* Add GIT_MKDIR_VERIFY_DIR to give an error if item is a file,
  not a dir (previously an EEXISTS error was ignored, even for
  files) and enable this flag for git_futils_mkpath2file call
* Improve accuracy of error messages from git_futils_mkdir