Adding support to update already defined event's attribute with
event modifier. This change will allow to use group modifier as
an update to the existing event modifiers.

Adding 'add' parameter to the parse_events__modifier_event function.
Calling it with 'add' = false/true, the event modifier is
initialized/updated respectively.

Added exclude_GH flag to evsel struct, because we need to remember
if one of 'GH' modifiers was used for event. The reason is that the
default settings for exclude_guest is 1 and during the group
modifier processing we have no other way of knowing if it was set
by default or by event modifier.

Keeping the current behaviour, that any event/group modifier reset
the defaults for exclude_host (0) and exclude_guest (1).
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ulrich Drepper <drepper@gmail.com>
Link: http://lkml.kernel.org/n/tip-8peaey3e2qc9dwtkvzbi4wmx@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>