Pass section when available to bfd_octets_per_byte
and other tidies. I think it's better to default to passing the
section to bfd_octets_per_byte, even in cases where we know it won't
make a difference.
A number of the coff reloc functions used bfd_octets_per_byte wrongly,
not factoring it into the offset into the data buffer. As it happens,
the targets using those files always had bfd_octets_per_byte equal to
one, so there wasn't any detectable wrong behaviour. However, it is
wrong in the source and might cause trouble for anyone creating a new
target. Besides fixing that, the patch also defines OCTETS_PER_BYTE
as one in target files where that is appropriate.