perf evsel: Introduce perf_evsel__{str,int}val methods

perf evsel: Introduce perf_evsel__{str,int}val methods

Wrappers to the libtraceevent routines, so that we can further reduce
the surface contact perf builtins have with it.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-rtmgzptvrifzjxqwb9vs6g1b@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>