Simplify handling of autotest tracing
* lib/autotest/general.m4 (at_fn_check_prepare_notrace): Use
$at_trace_echo. Add new REASON argument.
(at_fn_check_prepare_trace): Do not call at_fn_check_prepare_notrace.
Use $at_check_filter_trace.
(at_fn_check_prepare_dynamic): Use at_fn_check_prepare_notrace.
(at_traceon): Initialize to ':'.
(at_traceoff): Remove, use 'set +x' instead throughout.
(at_check_filter_trace, at_trace_echo): New shell variables.
Initialize them if tracing is requested.
(_AT_DECIDE_TRACEABLE): Adjust call to at_fn_check_prepare_notrace.