ELF core file size checks

Catch fuzzed segments where p_offset + p_filesz wraps, and limit error
output.

	* elfcore.h (elf_core_file_p): Rewrite segment checks using
	bfd_get_file_size.  Set read_only on file size errors.
	* elfcode.h (elf_swap_shdr_in): Don't repeat error message.