CODING_STYLE: mention that dup() should not be used

CODING_STYLE

@@ -232,3 +232,10 @@
   "return" to exit from the main function of a process. If you
   fork()ed off a child process, please use _exit() instead of exit(),
   so that the exit handlers are not run.
+
+- Please never use dup(). Use fcntl(fd, F_DUPFD_CLOEXEC, 3)
+  instead. For two reason: first, you want O_CLOEXEC set on the new fd
+  (see above). Second, dup() will happily duplicate your fd as 0, 1,
+  2, i.e. stdin, stdout, stderr, should those fds be closed. Given the
+  special semantics of those fds, it's probably a good idea to avoid
+  them. F_DUPFD_CLOEXEC with "3" as parameter avoids them.