com_err.h, error_table.h: Move definition of the error_table

com_err.h, error_table.h: Move definition of the error_table
	structure from the internal error_table.h to com_err.h,
	since it now needs to be public.

et_c.awk, et_h.awk: Import changes from krb5's et library so
	that the error_table structure is defined and available
	publically.

error_message.c: Import krb5 and heimdall com_err extensions to
	the et library.