Pass "plugin" as target for "ar --plugin" when opening a BFD file.

Pass "plugin" as target for "ar --plugin" when opening a BFD file.

2010-09-15  H.J. Lu  <hongjiu.lu@intel.com>

	* ar.c (plugin_target): New.
	(main): Set plugin_target to "plugin" for --plugin.
	(open_inarch): Initialize target to plugin_target.
	(replace_members): Pass plugin_target to ar_emul_replace
	and ar_emul_append.

	* binemul.h (ar_emul_append): Add a target argument.
	(ar_emul_default_append): Likewise.
	(ar_emul_replace): Likewise.
	(ar_emul_default_replace): Likewise.
	(bin_emulation_xfer_struct): Add a target argument to
	ar_append and ar_replace.

	* binemul.c (ar_emul_append): Updated.  Pass target to bfd_openr.
	(do_ar_emul_default_append): Likewise.
	(ar_emul_default_append): Likewise.
	(ar_emul_replace): Likewise.
	(ar_emul_default_replace): Likewise.