stpd kernel event filtering

[Mao, Bibo]

Sometimes when tracing kernel system call like sys_read()/sys_write(),
user maybe wants to filter kernel events generated by stapd. So in
tapset one function need be added:
	function stpd:long () %{
        THIS->__retvalue = _stp_pid;
	%}
And in the script comparison sentence need be hard-code in the script
	If (pid() != stpd())
		.......
I do not know whether there should be one additional option in stap. If
this option is on, all probepoint related with stpd will be discarded;
else user will get all real raw data including stpd process.

bibo,mao