seccomp: rework seccomp code, to improve compat with some archs

This substantially reworks the seccomp code, to ensure better
compatibility with some architectures, including i386.

So far we relied on libseccomp's internal handling of the multiple
syscall ABIs supported on Linux. This is problematic however, as it does
not define clear semantics if an ABI is not able to support specific
seccomp rules we install.

This rework hence changes a couple of things:

- We no longer use seccomp_rule_add(), but only
  seccomp_rule_add_exact(), and fail the installation of a filter if the
  architecture doesn't support it.

- We no longer rely on adding multiple syscall architectures to a single filter,
  but instead install a separate filter for each syscall architecture
  supported. This way, we can install a strict filter for x86-64, while
  permitting a less strict filter for i386.

- All high-level filter additions are now moved from execute.c to
  seccomp-util.c, so that we can test them independently of the service
  execution logic.

- Tests have been added for all types of our seccomp filters.

- SystemCallFilters= and SystemCallArchitectures= are now implemented in
  independent filters and installation logic, as they semantically are
  very much independent of each other.

Fixes: #4575

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