muvm was interpreting command arguments like '-c' (from /bin/sh -c)
as its own options. Adding '--' separates muvm options from the
command and its arguments.
D-Bus is supposed to (?) use it to decide whether it can use FD passing,
shared memory, etc. and while we do a lot of cross-domain magic it's not
quite seamless :) so let's not reuse the host one.
Instead of interpreting all that shell and running actual tmpfiles, use
a tiny stage before systemd that mounts a tmpfs at /run (preventing
systemd from doing the same), populates it with NixOS symlinks and
preserved resolv.conf, and mounts the immutable /etc overlay before
passing control over to systemd.
