perf tools: Add +field argument support for --sort option

Adding support to add field(s) to default sort order via using the '+'
prefix, like for report:

  $ perf report
  Samples: 2K of event 'cycles', Event count (approx.): 882172583
  Overhead  Command  Shared Object        Symbol
     7.39%  swapper  [kernel.kallsyms]    [k] intel_idle
     1.97%  firefox  libpthread-2.17.so   [.] pthread_mutex_lock
     1.39%  firefox  [snd_hda_intel]      [k] azx_get_position
     1.11%  firefox  libpthread-2.17.so   [.] pthread_mutex_unlock

  $ perf report -s +cpu
  Samples: 2K of event 'cycles', Event count (approx.): 882172583
  Overhead  Command  Shared Object        Symbol                  CPU
     2.89%  swapper  [kernel.kallsyms]    [k] intel_idle          000
     2.61%  swapper  [kernel.kallsyms]    [k] intel_idle          002
     1.20%  swapper  [kernel.kallsyms]    [k] intel_idle          001
     0.82%  firefox  libpthread-2.17.so   [.] pthread_mutex_lock  002

Works in general for commands using --sort option.

v2 with changes suggested:
  - Use dynamic memory instead static buffer
  - Fix error message typo

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jean Pihet <jean.pihet@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
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/20140823125948.GA1193@krava.brq.redhat.com


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