tracing: panic() if people try to use tracing, but TRACING_ON is not set.

Also clean things up so that help strings can more easily be added.
Move the help function into trace.py

src/python/m5/trace.py

@@ -0,0 +1,20 @@
+import internal
+import traceflags as flags
+
+from internal.trace import clear, output, set, ignore
+
+def enable():
+    internal.trace.cvar.enabled = True
+
+def help():
+    print "Base Flags:"
+    for flag in trace.flags.basic:
+        print "    %s: %s" % (flag, trace.flags.descriptions[flag])
+    print
+    print "Compound Flags:"
+    for flag in trace.flags.compound:
+        if flag == 'All':
+            continue
+        print "    %s: %s" % (flag, trace.flags.descriptions[flag])
+        print_list(trace.flags.compoundMap[flag], indent=8)
+        print