Implement objcopy/strip --keep-section=<sectionpattern>

Implement objcopy/strip --keep-section=<sectionpattern>

llvm-objcopy and llvm-strip support an option --keep-section that
keeps some sections from being removed.

	* objcopy.c (enum option_values): Add OPTION_KEEP_SECTION.
	(SECTION_CONTEXT_KEEP): Define.  Adjust other SECTION_CONTEXT macros.
	(copy_usage): Describe --keep-section.
	(strip_usage): Likewise.
	(copy_main): Handle SECTION_CONTEXT_KEEP.
	(strip_main): Likewise.
	(is_strip_section_1): Likewise.
	* testsuite/binutils-all/objcopy.exp: Add tests.
	* testsuite/binutils-all/keep-section-1.d: New test driver file.
	* testsuite/binutils-all/keep-section-2.d: Likewise.
	* doc/binutils.texi: Document the new feature.
	* NEWS: Mention the new feature.