Make _bfd_error_buf static

This makes _bfd_error_buf static and adds a way to clear it.  I felt
that this made the subsequent patches a little cleaner.

	* bfd.c (_bfd_error_buf): Now static.
	(bfd_set_input_error): Use _bfd_clear_error_data.
	(_bfd_clear_error_data): New function.
	(bfd_init): Use _bfd_clear_error_data.
	* libbfd.h: Regenerate.
	* opncls.c (bfd_close_all_done): Use _bfd_clear_error_data.
	* po/bfd.pot: Regenerate.