Add mkdtemp() to libpgport.

This function is pervasive on free software operating systems; import
NetBSD's implementation.  Back-patch to 8.4, like the commit that will
harness it.

src/include/port.h

@@ -462,6 +462,9 @@ extern int	pg_check_dir(const char *dir);
 /* port/pgmkdirp.c */
 extern int	pg_mkdir_p(char *path, int omode);
 
+/* port/mkdtemp.c */
+extern char *mkdtemp(char *path);
+
 /* port/pqsignal.c */
 typedef void (*pqsigfunc) (int signo);
 extern pqsigfunc pqsignal(int signo, pqsigfunc func);