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

. Print message at script start telling how to get te summary
. Print the syscall name using the audit-lib-python package, if
  installed
. Print the errno string
. Accept both pid (if numeric) or COMM name

Now it looks like this:

[root@emilia ~]# perf trace failed-syscalls-by-pid
Press control+C to stop and show the summary
^C
syscall errors:

comm [pid]                           count
------------------------------  ----------

automount [1670]
  syscall: futex
    err = ETIMEDOUT                     39

irqbalance [1462]
  syscall: openat
    err = ENOENT                         4

perf [7888]
  syscall: lseek
    err = ESPIPE                         1
  syscall: open
    err = ENOENT                        24

perf [7889]
  syscall: ioctl
    err = EINVAL                         1
  syscall: readlink
    err = EINVAL                         2
  syscall: open
    err = ENOENT                       389
  syscall: stat
    err = ENOENT                       141
  syscall: lseek
    err = ESPIPE                         3
[root@emilia ~]#

[root@emilia ~]# perf trace failed-syscalls-by-pid 1670
Press control+C to stop and show the summary
^C
syscall errors:

comm [pid]                           count
------------------------------  ----------

automount [1670]
  syscall: futex
    err = ETIMEDOUT                      2
[root@emilia ~]#
[root@emilia ~]#
[root@emilia ~]#
[root@emilia ~]# perf trace failed-syscalls-by-pid automount
Press control+C to stop and show the summary
^C
syscall errors for automount:

comm [pid]                           count
------------------------------  ----------

automount [1669]
  syscall: futex
    err = ETIMEDOUT                      1

automount [1670]
  syscall: futex
    err = ETIMEDOUT                      5
[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>