Change the behaviour of the --only-keep-debug option to objcopy and strip so that the sh_link and sh_info fields in stripped section headers are preserved.

bfd	* elf.c (_bfd_elf_copy_private_bfd_data): Copy the sh_link and
	sh_info fields of sections whose type has been changed to
	SHT_NOBITS.

bin	* doc/binutils.texi: Document that the --only-keep-debug option
	to strip and objcopy preserves the section headers of stripped
	sections.

tests	* binutils-all/objcopy.exp (keep_debug_symbols_and_check_links):
	New proc.  Checks that debug-info-only binaries retain the
	sh_link field in stripped sections.