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?

src/core/swap.c

@@ -781,7 +781,7 @@ static void swap_enter_activating(Swap *s) {
                 }
         }
 
-        r = exec_command_set(s->control_command, "/sbin/swapon", "--fixpgsz", NULL);
+        r = exec_command_set(s->control_command, SWAPON, "--fixpgsz", NULL);
         if (r < 0) {
                 log_unit_warning_errno(UNIT(s), r, "Failed to initialize swapon command line: %m");
                 goto fail;
@@ -826,7 +826,7 @@ static void swap_enter_deactivating(Swap *s) {
         s->control_command = s->exec_command + SWAP_EXEC_DEACTIVATE;
 
         r = exec_command_set(s->control_command,
-                             "/sbin/swapoff",
+                             SWAPOFF,
                              s->what,
                              NULL);
         if (r < 0) {