libc: Set the close-on-exec flag for directories opened using opendir()

This is apparently mandated by POSIX.

libc/src/dirent.cpp

@@ -20,6 +20,8 @@ extern "C"
             return nullptr;
         }
 
+        fcntl(fd, F_SETFD, FD_CLOEXEC);
+
         dp->_fd = fd;
 
         return dp;