Make coff backend data read-only

The bfd_coff_backend_data struct should be read-only, the only thing
preventing this is that objcopy writes to one of the fields,
_bfd_coff_long_section_names.  This patch creates a copy of the field
in bfd coff_obj_tdata, which makes more sense anyway.  When enabling
long section names the intent is to do so for a particular bfd, not
for all bfds that might happen to be using the target xvec.

bfd/
	* coffcode.h: Update coff long section name comment.
	(bfd_coff_set_long_section_names_allowed): Use macro accessor
	to set flag.
	(bfd_coff_set_long_section_names_disallowed): Tidy.
	(coff_backend_info): Return a const pointer.
	(bfd_coff_std_swap_table, ticoff0_swap_table, ticoff1_swap_table),
	(bigobj_swap_table): Make const.
	(bfd_coff_long_section_names): Use tdata copy.
	(coff_mkobject): Set long_section_names from coff_backend_info.
	* coff-go32.c (_bfd_go32_mkobject): Likewise.
	* peicode.h (pe_mkobject): Likewise.
	* coff-sh.c (bfd_coff_small_swap_table): Make const.
	* libcoff-in.h (struct coff_tdata): Add long_section_names,
	reorder fields.
	* libcoff.h: Regenerate.
binutils/
	* objcopy.c (set_long_section_mode): Move earlier in file.
	(copy_object): Call set_long_section_mode here, after setting
	output format.
	(copy_file): Don't call set_long_section_mode.