Seccomp filtering and the no-new-privileges functionality improve the security
of the sandbox, and have been enabled by default for a long time. In
https://git.lix.systems/lix-project/lix/issues/265 it was decided that they
should be enabled unconditionally. Accordingly, remove the allow-new-privileges
(which had weird behavior anyway) and filter-syscall settings, and force the
security features on. Syscall filtering can still be enabled at build time to
support building on architectures libseccomp doesn't support.
Change-Id: Iedbfa18d720ae557dee07a24f69b2520f30119cb
f047e4357b