PR10260: Clean up all resources after init errors

PR10260: Clean up all resources after init errors

When anything in systemtap_module_init fails, and we return non-zero,
then the module load is aborted.  The normal module unload path
(systemtap_module_exit) is not even attempted, so we need to make sure
that all partially-allocated resources are returned.

Our timer callbacks for the gettimeofday subsystem are a classic example
of this error.  If we don't unregister the timers before aborting init,
they will later be called and cause a kernel fault.

We also were neglecting to free the percpu context.  A memory leak is
less harmful, but that's fixed now too.