std: Handle DT_DIR file types in dirent pointers

This "fast path" in `DirEntry::file_type` on Unix wasn't turning out to be so
much of a fast path as the `DT_DIR` case wasn't handled, so directories fell
back to using `lstat` instead. This commit adds the missing case to return
quickly if a path is a directory and `DirEntry::file_type` is used.

Author: alexcrichton

src/rt/rust_builtin.c

@@ -58,6 +58,7 @@ rust_dir_get_mode(struct dirent* entry_ptr) {
         case DT_LNK: return S_IFLNK;
         case DT_REG: return S_IFREG;
         case DT_SOCK: return S_IFSOCK;
+        case DT_DIR: return S_IFDIR;
     }
 #endif
     return -1;