elflink.c: rename abfd and bed

This renames "abfd" function parameters to "obfd" when that parameter
is always the output bfd, and to "dynobj" when it is always the input
object used to attach dynamic sections.  "bed" variables which are set
to the output bfd or dynobj backend data are renamed to "obed".  (The
dynobj xvec must be the same as the output xvec, or bad things
happen.)  The idea is to make the use of bfd* parameter a little more
obvious when reading the source.