perf tools: Add basic event modifier sanity check

Updating event parser to allow any non zero string containing [ukhpGH]
characters for event modifier.

The modifier sanity is checked later in parse-event object logic.  The
check validates modifier to contain only one instance of any modifier
(apart from 'p') present.

v2:
  - added length check suggested Namhyung Kim

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
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>
Link: http://lkml.kernel.org/r/20121113143258.GA2481@krava.brq.redhat.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>