core: convert Slice= into a proper dependency (and add a back dependency)

The slice a unit is assigned to is currently a UnitRef reference. Let's
turn it into a proper dependency, to simplify and clean up code a bit.
Now that new dep types are cheaper, deps should generally be preferable
over everything else, if the concept applies.

This brings one major benefit: we often have to iterate through all unit
a slice contains. So far we iterated through all Before= dependencies of
the slice unit to achieve that, filtering out unrelated units, and
taking benefit of the fact that slice units are implicitly ordered
Before= the units they contain. By making Slice= a proper dependency,
and having an accompanying SliceOf= dependency type, this is much
simpler and nicer as we can directly enumerate the units a slice
contains.

The forward dependency is actually called InSlice internally, since we
already used the UNIT_SLICE name as UnitType field. However, since we
don't intend to expose the dependency to users as dep anyway (we already
have the regular Slice D-Bus property for this) this shouldn't matter.
The SliceOf= implicit dependency type (the erverse of Slice=/InSlice=)
is exported over the bus, to make things a bit nicer to debug and
discoverable.

src/basic/unit-def.c

@@ -282,6 +282,8 @@ static const char* const unit_dependency_table[_UNIT_DEPENDENCY_MAX] = {
         [UNIT_JOINS_NAMESPACE_OF] = "JoinsNamespaceOf",
         [UNIT_REFERENCES] = "References",
         [UNIT_REFERENCED_BY] = "ReferencedBy",
+        [UNIT_IN_SLICE] = "InSlice",
+        [UNIT_SLICE_OF] = "SliceOf",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(unit_dependency, UnitDependency);

src/basic/unit-def.h

@@ -245,6 +245,10 @@ typedef enum UnitDependency {
         UNIT_REFERENCES,              /* Inverse of 'references' is 'referenced_by' */
         UNIT_REFERENCED_BY,
 
+        /* Slice= */
+        UNIT_IN_SLICE,
+        UNIT_SLICE_OF,
+
         _UNIT_DEPENDENCY_MAX,
         _UNIT_DEPENDENCY_INVALID = -EINVAL,
 } UnitDependency;