service: rename Type=finish to Type=oneshot and allow multiple ExecStart= lines for...

service: rename Type=finish to Type=oneshot and allow multiple ExecStart= lines for oneshot services

In contrast to the other service types oneshot services are usually not
long lasting and there's not necessarily a single clean main process for
them. This change allows multiple ExecStart= lines for this type of
services so that the admin/developer doesn't have to arbitrarily pick on
of various sequential commands as the "main one".