<elf.h> should make it easier to port our ELF-related tools to other Unices.

For the moment it only includes <sys/exec_elf.h> but the goal is to
stop pulling it directly and also replace <elf_abi.h> at least for
base applications.
ok deraadt@, jasper@, naddy@

src/include/Makefile

@@ -1,4 +1,4 @@
-#	$OpenBSD: Makefile,v 1.221 2017/10/15 23:40:33 guenther Exp $
+#	$OpenBSD: Makefile,v 1.222 2017/10/17 09:34:52 mpi Exp $
 #	$NetBSD: Makefile,v 1.59 1996/05/15 21:36:43 jtc Exp $
 
 #	@(#)Makefile	5.45.1.1 (Berkeley) 5/6/91
@@ -13,7 +13,7 @@ FILES=	a.out.h ar.h asr.h assert.h \
 	bitstring.h blf.h bsd_auth.h \
 	complex.h cpio.h ctype.h curses.h \
 	db.h dirent.h disktab.h dlfcn.h \
-	elf_abi.h err.h errno.h \
+	elf.h elf_abi.h err.h errno.h \
 	fenv.h float.h fnmatch.h fstab.h fts.h ftw.h \
 	getopt.h glob.h grp.h \
 	icdb.h ieeefp.h ifaddrs.h inttypes.h iso646.h \

src/include/elf.h

@@ -0,0 +1,12 @@
+/*	$OpenBSD: elf.h,v 1.1 2017/10/17 09:34:52 mpi Exp $	*/
+
+/*
+ * Public domain.
+ */
+
+#ifndef _ELF_H_
+#define _ELF_H_
+
+#include <sys/exec_elf.h>
+
+#endif /* _ELF_H_ */