seccomp code review

[jnovy]

There is broken logic in conmon.c USE_SECCOMP macro usage and seccomp_notify.c as found in issue #582. The purpose of this ticket is to investigate how we move forward with seccomp support in conmon.

[jnovy]

Related build issue: #275

[jnovy]

Following up on #606 (comment) - please let me know which bits we want to remove @giuseppe