Sync our copy of the timezone library with IANA release tzcode2016j.

This is a trivial update (consisting in fact only in the addition of
a comment).  The point is just to get back to being synced with an
official release of tzcode, rather than some ad-hoc point in their
commit history, which is where commit 1f87181e1 left it.

src/timezone/zic.c

@@ -3634,6 +3634,11 @@ mkdirs(char const * argname, bool ancestors)
 		 */
 		if (mkdir(name, MKDIR_UMASK) != 0)
 		{
+			/*
+			 * For speed, skip itsdir if errno == EEXIST.  Since mkdirs is
+			 * called only after open fails with ENOENT on a subfile, EEXIST
+			 * implies itsdir here.
+			 */
 			int			err = errno;
 
 			if (err != EEXIST && !itsdir(name))