systemd: do not use host system-uid/gid ranges

systemd: do not use host system-uid/gid ranges

systemd meson.build uses values from host /etc/login.defs if
system-uid-max and system-gid-max build options are not explicitly
specified.

Avoid that by setting system-uid-max and system-gid-max to 999 which is
the systemd default if SYSTEM_UID_MAX and SYSTEM_GID_MAX are not set in
/etc/login.defs.

Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>