pid1: propagate the original command line when reexecuting

When we reexec the manager in a container, we lose configuration settings on
the kernel command line:

  $ systemd-nspawn -M rawhide -b systemd.status-unit-format=name systemd.show-status=yes
  ...
  # tr '\0' ' ' </proc/1/cmdline
  /usr/lib/systemd/systemd systemd.status_unit_format=combined systemd.show-status=yes
  # sudo systemctl daemon-reexec
  # tr '\0' ' ' </proc/1/cmdline
  /usr/lib/systemd/systemd --system --deserialize 20

  This means that after daemon-reexec, the settings that we gain from the
  commandline are reset to defaults.

So let's reeexecute with the original arguments copied over, modulo some
filtering.