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.

(cherry picked from commit 846f1da4)
(cherry picked from commit f3af6ba8)