tracing/boot: Support multiple handlers for per-event histogram

Support multiple handlers for per-event histogram in boot-time tracing.
Since the histogram can register multiple same handler-actions with
different parameters, this expands the syntax to support such cases.

With this update, the 'onmax', 'onchange' and 'onmatch' handler subkeys
under per-event histogram option will take a number subkeys optionally
as below. (see [.N])

ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist {
     onmax|onchange[.N] { var = <VAR>; <ACTION> [= <PARAM>] }
     onmatch[.N] { event = <EVENT>; <ACTION> [= <PARAM>] }
}

The 'N' must be a digit (or digit started word).

Thus user can add several handler-actions to the histogram,
for example,

ftrace.event.SOMEGROUP.SOMEEVENT.hist {
   keys = SOME_ID; lat = common_timestamp.usecs-$ts0
   onmatch.1 {
	event = GROUP1.STARTEVENT1
	trace = latency_event, SOME_ID, $lat
   }
   onmatch.2 {
	event = GROUP2.STARTEVENT2
	trace = latency_event, SOME_ID, $lat
   }
}

Then, it can trace the elapsed time from GROUP1.STARTEVENT1 to
SOMEGROUP.SOMEEVENT, and from GROUP2.STARTEVENT2 to
SOMEGROUP.SOMEEVENT with SOME_ID key.

Link: https://lkml.kernel.org/r/162856124905.203126.14913731908137885922.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>