stage-2: parameterized systemd executable

This lets users do sneaky things before systemd starts, and
permanently affect the environment in which systemd runs. For example,
we could start systemd in a non-default network namespace by setting
the systemdExecutable to a wrapper script containing:

    #!/bin/sh
    ip netns add virtual
    touch /var/run/netns/physical
    mount -o bind /proc/self/ns/net /var/run/netns/physical
    exec ip netns exec virtual systemd

_note: the above example does literally work, but there are unresolved
problems with udev and dhcp._

nixos/modules/system/boot/stage-2-init.sh

@@ -169,4 +169,4 @@ exec {logOutFd}>&- {logErrFd}>&-
 echo "starting systemd..."
 PATH=/run/current-system/systemd/lib/systemd:@fsPackagesPath@ \
     LOCALE_ARCHIVE=/run/current-system/sw/lib/locale/locale-archive \
-    exec systemd
+    exec @systemdExecutable@

nixos/modules/system/boot/stage-2.nix

@@ -10,6 +10,7 @@ let
     src = ./stage-2-init.sh;
     shellDebug = "${pkgs.bashInteractive}/bin/bash";
     shell = "${pkgs.bash}/bin/bash";
+    inherit (config.boot) systemdExecutable;
     isExecutable = true;
     inherit (config.nix) readOnlyStore;
     inherit useHostResolvConf;
@@ -72,6 +73,15 @@ in
         '';
       };
 
+      systemdExecutable = mkOption {
+        default = "systemd";
+        type = types.str;
+        description = ''
+          The program to execute to start systemd. Typically
+          <literal>systemd</literal>, which will find systemd in the
+          PATH.
+        '';
+      };
     };
 
   };