Add option to objdump/readelf to disable access to debuginfod servers.

Add option to objdump/readelf to disable access to debuginfod servers.

	* dwarf.c (use_debuginfod): New variable.  Set to 1.
	(load_separate_debug_info): Only call
	debuginfod_fetch_separate_debug_info is use_debuginfod is true.
	(dwarf_select_sections_by_names): Add do-not-use-debuginfod and
	use-debuginfod options.
	(dwarf_select_sections_by_letters): Add D and E options.
	* dwarf.h (use_debuginfod): New extern.
	* objdump.c (usage): Mention the new options.
	* readelf.c (usage): Likewise.
	* doc/binutils.texi: Document the new options.
	* doc/debug-options.texi: Describe the new options.
	* NEWS: Mention the new feature.
	* testsuite/binutils-all/debuginfod.exp: Add tests of the new
	options.