unit: add DefaultDependencies= setting

unit: add DefaultDependencies= setting

In order to simplify writing of unit files introduce default
dependencies that are added to all units unless explictly disabled in a
unit. This option can be switched off for select units that are involved
in early boot-up ot late system shutdown,

This should simplify service files for most normal daemons, but breaks
existing service files for software involved in early boot (notably
udev), which need to be updated for a DefaultDependencies=no setting)