Merge pull request #16819 from keszybz/seccomp-enosys

Return ENOSYS in nspawn for "unknown" syscalls

diff --cc man/systemd.exec.xml
Simple merge

diff --cc meson.build
Simple merge

diff --cc src/shared/meson.build
Simple merge

diff --cc src/shared/seccomp-util.c
Simple merge

diff --cc src/test/test-seccomp.c
Simple merge