Change the readelf and objdump programs so that they will automatically follow links...

Change the readelf and objdump programs so that they will automatically follow links to separate debug info files.

	* configure.ac (follow-debug-links): Add option to enable or
	disable the following of debug links by default.  Set the
	default for the option to be 'follow'.
	* dwarf.c (do_follow_links): Initialise with DEFAULT_FOR_FOLLOW_LINKS.
	(dwarf_select_sections_by_names): Add no-follow-links option.
	(dwarf_select_sections_by_letter): Add 'N' option.
	* objdump.c (usage): Add conditional text describing the
	follow links option.
	(slurp_symtab): Ensure that there is a NULL entry at the end
	of the symbol table.
	(slurp_dynamic_symtab): Likewise.
	(dump_bfd): When extending the symbol table, ensure that there
	is still a NULL entry at the end.
	* readelf.c (usage): Add conditional text describing the
	follow links option.
	* doc/binutils.texi: Update documentation for objcopy and
	readelf.
	* doc/debug.options.texi: Update documentation of the
	follow-links option.
	* config.in: Regenerate.
	* configure: Regenerate.
	* testsuite/binutils-all/compress.exp: Add the -WN option to
	objdump command lines that are not expecting to follow links.
	* testsuite/binutils-all/readelf.exp: Add the
	--debug-dump=no-follow-links option to tests that are not
	expecting to follow debug links.

gas	* testsuite/gas/mach-o/sections-1.d: Stop automatic debug link
        following.
	* testsuite/gas/xgate/insns-dwarf2.d: Likewise.

ld	* testsuite/ld-elf/sec64k.exp: Stop readelf from automatically
	following debug links.