Sort the output according to ASCII character list (using strcmp), which
supports both number sequence and alphabet sequence.

Example:

Before this patch:

 $ perf list

 List of pre-defined events (to be used in -e):
   cpu-cycles OR cycles                               [Hardware event]
   instructions                                       [Hardware event]
   cache-references                                   [Hardware event]
   cache-misses                                       [Hardware event]
   branch-instructions OR branches                    [Hardware event]
   branch-misses                                      [Hardware event]
   bus-cycles                                         [Hardware event]
   ...                                                ...

   jbd2:jbd2_start_commit                             [Tracepoint event]
   jbd2:jbd2_commit_locking                           [Tracepoint event]
   jbd2:jbd2_run_stats                                [Tracepoint event]
   block:block_rq_issue                               [Tracepoint event]
   block:block_bio_complete                           [Tracepoint event]
   block:block_bio_backmerge                          [Tracepoint event]
   block:block_getrq                                  [Tracepoint event]
   ...                                                ...

After this patch:

 $ perf list

 List of pre-defined events (to be used in -e):
   branch-instructions OR branches                    [Hardware event]
   branch-misses                                      [Hardware event]
   bus-cycles                                         [Hardware event]
   cache-misses                                       [Hardware event]
   cache-references                                   [Hardware event]
   cpu-cycles OR cycles                               [Hardware event]
   instructions                                       [Hardware event]
   ...                                                ...

   block:block_bio_backmerge                          [Tracepoint event]
   block:block_bio_complete                           [Tracepoint event]
   block:block_getrq                                  [Tracepoint event]
   block:block_rq_issue                               [Tracepoint event]
   jbd2:jbd2_commit_locking                           [Tracepoint event]
   jbd2:jbd2_run_stats                                [Tracepoint event]
   jbd2:jbd2_start_commit                             [Tracepoint event]
   ...                                                ...
Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1425032491-20224-2-git-send-email-yunlong.song@huawei.com
[ Don't forget closedir({sys,evt}_dir) when handling errors ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>