perf python scripting: Improve the syscalls-counts script

perf python scripting: Improve the syscalls-counts script

. Print message at script start telling how to get te summary
. Print the syscall name

Now it looks like this:

[root@emilia ~]# perf trace syscall-counts
Press control+C to stop and show the summary
^C
syscall events:

event                                          count
----------------------------------------  -----------
read                                          102752
open                                            1293
close                                            878
write                                            319
stat                                             185
fstat                                            149
getdents                                         116
mmap                                              98
brk                                               80
rt_sigaction                                      66
munmap                                            42
mprotect                                          24
lseek                                             21
lstat                                              7
rt_sigprocmask                                     4
futex                                              3
statfs                                             3
ioctl                                              3
readlink                                           2
select                                             2
getegid                                            1
geteuid                                            1
getgid                                             1
getuid                                             1
getrlimit                                          1
fcntl                                              1
uname                                              1
[root@emilia ~]#

Cc: David S. Miller <davem@davemloft.net>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>