Preserve a few more bfd fields in check_format_matches

Preserve a few more bfd fields in check_format_matches

AOUT and COFF targets set symcount and start_address in their object_p
functions.  If these are used anywhere then it would pay to save and
restore them so that a successful match gets the values expected
rather than that for a later unsuccessful target match.

	* format.c (struct bfd_preserve): Move some fields.  Add
	symcount, read_only and start_address.
	(bfd_preserve_save): Save..
	(bfd_preserve_restore): ..and restore..
	(bfd_reinit): ..and zero new fields.