Don't include libbfd.h outside of bfd, part 6

Some messing with plugin code in order to not need arelt_size in
ld code.  File descriptor handling in ld/plugin.c is tidied too,
simply duping the open fd rather than opening the file again.

bfd/
	* elflink.c: Include plugin-api.h.
	* plugin.c (bfd_plugin_open_input): New function, extracted from..
	(try_claim): ..here.
	* plugin.h: Don't include bfd.h.
	(bfd_plugin_open_input): Declare.
binutils/
	* ar.c: Include plugin-api.h.
	* nm.c: Likewise.
ld/
	* plugin.c: Don't include libbfd.h.  Include plugin-api.h
	before bfd/plugin.h.
	(plugin_object_p): Use bfd_plugin_open_input.