tracing: Add hist trigger support for pausing and continuing a trace

Allow users to append 'pause' or 'continue' to an existing trigger in
order to have it paused or to have a paused trace continue.

This expands the hist trigger syntax from this:
    # echo hist:keys=xxx:vals=yyy:sort=zzz.descending \
          [ if filter] >> event/trigger

to this:

    # echo hist:keys=xxx:vals=yyy:sort=zzz.descending:pause or cont \
          [ if filter] >> event/trigger

Link: http://lkml.kernel.org/r/b672a92c14702cb924cdf6fc27ea1809bed04907.1457029949.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>