Add macro for checking if some flags are set

This way we don't need to repeat the argument twice.
I didn't replace all instances. I think it's better to leave out:
- asserts
- comparisons like x & y == x, which are mathematically equivalent, but
  here we aren't checking if flags are set, but if the argument fits in the
  flags.

src/core/cgroup.c

@@ -1303,7 +1303,7 @@ const char *unit_get_realized_cgroup_path(Unit *u, CGroupMask mask) {
 
                 if (u->cgroup_path &&
                     u->cgroup_realized &&
-                    (u->cgroup_realized_mask & mask) == mask)
+                    FLAGS_SET(u->cgroup_realized_mask, mask))
                         return u->cgroup_path;
 
                 u = UNIT_DEREF(u->slice);

src/core/unit.c

@@ -1053,7 +1053,7 @@ static void print_unit_dependency_mask(FILE *f, const char *kind, UnitDependency
                 if (mask == 0)
                         break;
 
-                if ((mask & table[i].mask) == table[i].mask) {
+                if (FLAGS_SET(mask, table[i].mask)) {
                         if (*space)
                                 fputc(' ', f);
                         else
@@ -2695,8 +2695,8 @@ static int unit_add_dependency_hashmap(
         if (info.data) {
                 /* Entry already exists. Add in our mask. */
 
-                if ((info.origin_mask & origin_mask) == info.origin_mask &&
-                    (info.destination_mask & destination_mask) == info.destination_mask)
+                if (FLAGS_SET(origin_mask, info.origin_mask) &&
+                    FLAGS_SET(destination_mask, info.destination_mask))
                         return 0; /* NOP */
 
                 info.origin_mask |= origin_mask;