Make bfd_error_handler_type like vprintf

Make bfd_error_handler_type like vprintf

It was like printf, which means you can't use bfd_set_error_handler to
hook in a function to do something and then call the original handler.

The patch also deletes some unused functions and makes pointers local.

bfd/
	* bfd-in.h: Include stdarg.h.
	* bfd.c (bfd_error_handler_type): Make like vprintf.
	(_bfd_error_internal): Rename from _bfd_error_handler.  Make static.
	(error_handler_internal): New function, split out from..
	(_bfd_default_error_handler): ..here.  Rename to _bfd_error_handler.
	(bfd_set_error_handler): Update.
	(bfd_get_error_handler, bfd_get_assert_handler): Delete.
	(_bfd_assert_handler): Make static.
	* coffgen.c (null_error_handler): Update params.
	* elf-bfd.h (struct elf_backend_data <link_order_error_handler>):
	Don't use bfd_error_handler_type.
	* elf64-mmix.c (mmix_dump_bpo_gregs): Likewise.
	* elfxx-target.h (elf_backend_link_order_error_handler): Default
	to _bfd_error_handler.
	* libbfd-in.h (_bfd_default_error_handler): Don't declare.
	(bfd_assert_handler_type): Likewise.
	(_bfd_error_handler): Update.
	* bfd-in2.h: Regenerate.
	* libbfd.h: Regenerate.
ld/
	* ldlang.c (ignore_bfd_errors): Update params.