Add r_ignore to coff internal_reloc

Add a means of controlling _bfd_coff_generic_relocate_section for
cases where a backend relocate_section has already handled the reloc.

include/
	* coff/internal.h (struct internal_reloc): Reorganise.  Make
	r_type, r_size and r_extern bitfields.  Add r_ignore bitfield.
bfd/
	* coff-alpha.c (alpha_ecoff_swap_reloc_in): Init all fields of
	struct internal_reloc.
	* coff-mips.c (mips_ecoff_swap_reloc_in): Likewise.  Remove
	unnecessary casts.
	* coffswap.h (coff_swap_reloc_in): Likewise.
	* peicode.h (coff_swap_reloc_in): Likewise.
	* coffcode.h (coff_slurp_reloc_table): Don't clear r_offset.
	* cofflink.c (_bfd_coff_generic_relocate_section): Skip relocs
	with r_ignore set.