Clean up the handling of the M4 builtins tracing exception.

Clean up the handling of the M4 builtins tracing exception.
* bin/autom4te.in (Request::request): Don't complete M4 builtins
trace requests.
(@m4_builtins): Rename as...
(@m4_builtin): this.
(%m4_builtin_alternate_name): New.
(&parse_args): Complete the trace requests with alternate names.
(&handle_traces): Hence no longer do it here.
(&trace_requests): Remove, unused.