Fix an abort triggered when objcopy is used to set the "share" section flag on an...

Fix an abort triggered when objcopy is used to set the "share" section flag on an ELF section.

binutils* objcopy.c (check_new_section_flags): New function.  Reject the
	SEC_COFF_SHARED flag if the target is not a COFF binary.
	(copy_object): Call check_new_section_flags.
	(setup_section): Likewise.
	* doc/binutils.texi (objcopy): Add a note that the 'share' section
	flag cannot be applied to ELF binaries.

bfd	* elf.c (_bfd_elf_set_section_contents): Replace call to abort
	with error messages and failure return values.