when the log level is TRACE, this patch export the seccomp
filter to log file.

the ouput of `seccomp_export_pfc()` is human readable and this feature
is useful for user to make sure their `seccomp configuration file` is
right.

Output for he default ubuntu container's seccomp filter is the
following:

```
      lxc-start ubuntu 20170520024159.412 INFO     lxc_apparmor - lsm/apparmor.c:apparmor_process_label_set:238 - changed apparmor profile to lxc-container-default-cgns
if ($arch == 3221225534)
  # filter for syscall "finit_module" (313) [priority: 65535]
  if ($syscall == 313)
    action ERRNO(1);
  # filter for syscall "open_by_handle_at" (304) [priority: 65535]
  if ($syscall == 304)
    action ERRNO(1);
  # filter for syscall "kexec_load" (246) [priority: 65535]
  if ($syscall == 246)
    action ERRNO(1);
  # filter for syscall "delete_module" (176) [priority: 65535]
  if ($syscall == 176)
    action ERRNO(1);
  # filter for syscall "init_module" (175) [priority: 65535]
  if ($syscall == 175)
    action ERRNO(1);
  # filter for syscall "umount2" (166) [priority: 65533]
  if ($syscall == 166)
    if ($a1.hi32 & 0x00000000 == 0)
      if ($a1.lo32 & 0x00000001 == 1)
        action ERRNO(13);
  # default action
  action ALLOW;
if ($arch == 1073741827)
  # filter for syscall "finit_module" (350) [priority: 65535]
  if ($syscall == 350)
    action ERRNO(1);
  # filter for syscall "open_by_handle_at" (342) [priority: 65535]
  if ($syscall == 342)
    action ERRNO(1);
  # filter for syscall "kexec_load" (283) [priority: 65535]
  if ($syscall == 283)
    action ERRNO(1);
  # filter for syscall "delete_module" (129) [priority: 65535]
  if ($syscall == 129)
    action ERRNO(1);
  # filter for syscall "init_module" (128) [priority: 65535]
  if ($syscall == 128)
    action ERRNO(1);
  # filter for syscall "umount2" (52) [priority: 65534]
  if ($syscall == 52)
    if ($a1 & 0x00000001 == 1)
      action ERRNO(13);
  # default action
  action ALLOW;
if ($arch == 3221225534)
  # filter for syscall "kexec_load" (1073742352) [priority: 65535]
  if ($syscall == 1073742352)
    action ERRNO(1);
  # filter for syscall "finit_module" (1073742137) [priority: 65535]
  if ($syscall == 1073742137)
    action ERRNO(1);
  # filter for syscall "open_by_handle_at" (1073742128) [priority: 65535]
  if ($syscall == 1073742128)
    action ERRNO(1);
  # filter for syscall "delete_module" (1073742000) [priority: 65535]
  if ($syscall == 1073742000)
    action ERRNO(1);
  # filter for syscall "init_module" (1073741999) [priority: 65535]
  if ($syscall == 1073741999)
    action ERRNO(1);
  # filter for syscall "umount2" (1073741990) [priority: 65534]
  if ($syscall == 1073741990)
    if ($a1 & 0x00000001 == 1)
      action ERRNO(13);
  # default action
  action ALLOW;
action KILL;
      lxc-start ubuntu 20170520024159.412 NOTICE   lxc_start - start.c:start:1470 - Exec'ing "/sbin/init".
```
Signed-off-by: 0x0916 <w@laoqinren.net>