v0.4.0

Changed

• Seccomp is now activated via the seccomp syscall, not prctl

Added

• A new Error::Seccomp variant is added to indictate seccomp syscall failures
• Add apply_filter_all_threads convenience function which uses the seccomp
  TSYNC feature to synchronize all threads in the process to the same filter
• A new Error::ThreadSync variant is added to indicate failure to sync threads