environ and __progname are not declared in a public header; declare them

in libc's hidden/stdlib.h instead of in each .c file that needs one
ok deraadt@ gsoares@ mpi@

src/lib/libc/stdlib/getenv.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: getenv.c,v 1.11 2015/09/13 08:31:47 guenther Exp $ */
+/*	$OpenBSD: getenv.c,v 1.12 2016/03/13 18:34:21 guenther Exp $ */
 /*
  * Copyright (c) 1987, 1993
  *	The Regents of the University of California.  All rights reserved.
@@ -45,7 +45,6 @@
 char *
 __findenv(const char *name, int len, int *offset)
 {
-	extern char **environ;
 	int i;
 	const char *np;
 	char **p, *cp;

src/lib/libc/stdlib/malloc.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: malloc.c,v 1.182 2016/02/25 00:38:51 deraadt Exp $	*/
+/*	$OpenBSD: malloc.c,v 1.183 2016/03/13 18:34:21 guenther Exp $	*/
 /*
  * Copyright (c) 2008, 2010, 2011 Otto Moerbeek <otto@drijf.net>
  * Copyright (c) 2012 Matthew Dempsky <matthew@openbsd.org>
@@ -208,8 +208,6 @@ static int	malloc_active;		/* status of malloc */
 
 static u_char getrbyte(struct dir_info *d);
 
-extern char	*__progname;
-
 #ifdef MALLOC_STATS
 void malloc_dump(int);
 PROTO_NORMAL(malloc_dump);

src/lib/libc/stdlib/setenv.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: setenv.c,v 1.16 2015/09/13 08:31:47 guenther Exp $ */
+/*	$OpenBSD: setenv.c,v 1.17 2016/03/13 18:34:21 guenther Exp $ */
 /*
  * Copyright (c) 1987 Regents of the University of California.
  * All rights reserved.
@@ -32,7 +32,6 @@
 #include <stdlib.h>
 #include <string.h>
 
-extern char **environ;
 static char **lastenv;				/* last value of environ */
 
 /*

src/lib/libc/stdlib/system.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: system.c,v 1.11 2015/10/23 04:44:41 guenther Exp $ */
+/*	$OpenBSD: system.c,v 1.12 2016/03/13 18:34:21 guenther Exp $ */
 /*
  * Copyright (c) 1988 The Regents of the University of California.
  * All rights reserved.
@@ -36,8 +36,6 @@
 #include <unistd.h>
 #include <paths.h>
 
-extern char **environ;
-
 int
 system(const char *command)
 {