Simplify handling of autotest tracing

Simplify handling of autotest tracing

This patch simplifies at_fn_check_prepare_trace by noting that it
need not call at_fn_check_prepare_notrace.  Instead we can make
$at_traceon be ':' or 'set -x' instead of being empty when tracing
is disabled.

A similar approach is used for other variables set by
at_fn_check_prepare_trace and to conditionalize the message
about disabling tracing in at_fn_check_prepare_dynamic.

In addition, I changed $at_traceoff to 'set +x' throughout and
eliminated the variable.

2009-07-12  Paolo Bonzini  <bonzini@gnu.org>

	* 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.