core: add UNIT_GET_SLICE() helper

In a later commit we intend to move the slice logic to use proper
dependencies instead of a "UnitRef" object. This preparatory commit
drops direct use of the slice UnitRef object for a static inline
function UNIT_GET_SLICE() that is both easier to grok, and allows us to
easily replace its internal implementation later on.

src/core/slice.c

@@ -53,7 +53,7 @@ static int slice_add_parent_slice(Slice *s) {
 
         assert(s);
 
-        if (UNIT_ISSET(u->slice))
+        if (UNIT_GET_SLICE(u))
                 return 0;
 
         r = slice_build_parent_slice(u->id, &a);
@@ -101,7 +101,7 @@ static int slice_verify(Slice *s) {
         if (r < 0)
                 return log_unit_error_errno(UNIT(s), r, "Failed to determine parent slice: %m");
 
-        if (parent ? !unit_has_name(UNIT_DEREF(UNIT(s)->slice), parent) : UNIT_ISSET(UNIT(s)->slice))
+        if (parent ? !unit_has_name(UNIT_GET_SLICE(UNIT(s)), parent) : !!UNIT_GET_SLICE(UNIT(s)))
                 return log_unit_error_errno(UNIT(s), SYNTHETIC_ERRNO(ENOEXEC), "Located outside of parent slice. Refusing.");
 
         return 0;