Some post unmerged-usr cleanups (#38696)

Some post unmerged-usr  cleanups (#38696)

I noticed in our NixOS packaging that we were working around the fact
that core/swap.c looks for swapon and swapoff in /sbin

Lets make it configurable just like all the other util-linux binaries
through meson and make it default to /usr/sbin/{swapon,swapoff}

This way mounts work on a systemd without the /sbin -> /usr/sbin
compatibility symlink. (And as a side-effect has NixOS be able to have
it in /nix/store too like the other util-linux tools).

Given that `unmerged-usr` support was dropped in 255 I think this is a
safe change?