support table printing and histogram

simple script to use this new feature:

syscalls_histogram.kp
---------------------------------------------
hist = {}

function eventfun (e) {
        local syscall_name = e.name
        hist[syscall_name] = hist[syscall_name] + 1
}

os.probe("tp:syscalls", eventfun)

os.probe_end(function () {
        histogram(hist)
})

---------------------------------------------

^C
              value ------------- Distribution ------------- count
     sys_enter_ioctl |                                       1
      sys_exit_ioctl |                                       1
       sys_exit_poll |                                       10
 sys_exit_rt_siga... |                                       2
 sys_enter_rt_sig... |@@@@@@                                 74
 sys_enter_rt_sig... |                                       2
      sys_enter_poll |                                       10
     sys_enter_write |@@@                                    35
    sys_enter_select |@@@@@                                  59
  sys_exit_nanosleep |                                       2
 sys_enter_nanosleep |                                       2
 sys_exit_rt_sigp... |@@@@@@                                 74
     sys_exit_select |@@@@@                                  59
       sys_exit_read |@@@                                    35
      sys_exit_write |@@@                                    35
      sys_enter_read |@@@                                    35
[root@jovi ktap]#
add scripts/syscalls_histogram.kp