syscallfilter: port to libseccomp
core: rework syscall filter

- Allow configuration of an errno error to return from blacklisted
  syscalls, instead of immediately terminating a process.

- Fix parsing logic when libseccomp support is turned off

- Only keep the actual syscall set in the ExecContext, and generate the
  string version only on demand.
core: fix build without libseccomp