core/swap: /sbin -> /usr/sbin

core/swap: /sbin -> /usr/sbin

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).