postinst: handle masked units

postinst: handle masked units

check if a unit is masked before starting/restarting/reloading it,
as else we get pretty ugly error messages during upgrade.

as "deb-systemd-helper --quiet was-enabled" differs from the
"systemctl is-enabled" behaviour, the former returns true for masked
units while the latter does not, we have to manually call systemctl,
circumventing the deb helper.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Acked-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>