stage1: lessen systemd-nspawn dependency on host runtime

 * Introduces "/fakesdboot.so" shim for intercepting lstat of
   "/run/systemd/system" in stage1 rootfs
 * LD_PRELOAD=fakesdboot.so in stage1 systemd-nspawn execution
 * LD_LIBRARY_PATH=usr/lib of stage1 so we use stage1 libs
 * explicitly use stage1's ld-linux interpreter in executing
   systemd-nspawn

 It would be tidier to just chroot into stage1's rootfs before
 executing /init, but then we'd need to wrangle the bind mount
 source paths into the chroot beforehand.

 I'm leaning towards just rolling the systemd-nspawn
 functionality we actually desire into the statically-linked
 /init and remove systemd-nspawn from the bootstrap altogether as
 the next step.