perf python scripting: Improve the syscalls-by-pid script

perf python scripting: Improve the syscalls-by-pid script

. Print message at script start telling how to get te summary
. Print the syscall names
. Accept both pid (if numeric) or COMM name

Now it looks like this:

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

comm [pid]/syscalls                            count
----------------------------------------  ----------

automount [1670]
  futex                                            2

sshd [2322]
  rt_sigprocmask                                   4
  select                                           2
  write                                            1
  read                                             1

perf [15178]
  read                                          2506
  open                                           794
  close                                          769
  write                                          240
  getdents                                       112
  lseek                                           16
  stat                                             9
  perf_counter_open                                5
  fcntl                                            5
  mmap                                             5
  statfs                                           2

perf [15179]
  read                                         56701
  open                                           499
  stat                                           176
  fstat                                          149
  close                                          109
  mmap                                            98
  brk                                             75
  rt_sigaction                                    66
  munmap                                          42
  mprotect                                        24
  lstat                                            7
  lseek                                            5
  getdents                                         4
  ioctl                                            3
  readlink                                         2
  futex                                            1
  statfs                                           1
  getegid                                          1
  geteuid                                          1
  getgid                                           1
  getuid                                           1
  getrlimit                                        1
  fcntl                                            1
  uname                                            1
  write                                            1
[root@emilia tmp]# fg
-bash: fg: current: no such job
[root@emilia tmp]# perf trace syscall-counts-by-pid 2322
Press control+C to stop and show the summary
^C
syscall events by comm/pid:

comm [pid]/syscalls                            count
----------------------------------------  ----------

sshd [2322]
  rt_sigprocmask                                   4
  select                                           2
  write                                            1
  read                                             1
[root@emilia tmp]# perf trace syscall-counts-by-pid sshd
Press control+C to stop and show the summary
^C
syscall events for sshd:

comm [pid]/syscalls                            count
----------------------------------------  ----------

sshd [2322]
  rt_sigprocmask                                   4
  select                                           2
  write                                            1
  read                                             1
[root@emilia tmp]#

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>