This patch add function `get_action_name`, so we can print action name
in the log file. for example:

```
lxc-start ubuntu 20170515095416.561 INFO     lxc_seccomp - seccomp.c:parse_config_v2:613 - Adding compat rule for reject_force_umount action 0(kill).
lxc-start ubuntu 20170515095416.562 INFO     lxc_seccomp - seccomp.c:parse_config_v2:613 - Adding compat rule for kexec_load action 327681(errno).
```
Signed-off-by: 0x0916 <w@laoqinren.net>