analyze: show ungrouped syscalls separately (#17343)

This updates the "systemd-analyze syscall-filter" command to show a
special section of syscalls that are included in @known but in no other
group. Typically this should show syscalls we either should add to any
of the existing groups or where we unsure were they best fit in.

Right now, it mostly shows arch-specific compat syscalls, we probably
should move "@obsolete". This patch doesn't add thta however.