core: introduce exec_context_with_rootfs_strict() as a stricter version of exec_context_with_rootfs()

We have two very similar checks in place: in some contexts we want to
know if *any* RootDirectory= is configured, in the other we want to
suppress if it is configured to our regular root. Let's add a helper for
both (even if we only need it once), to make the mirrored behaviour
clear.

src/core/execute.c

@@ -2063,6 +2063,16 @@ bool exec_context_with_rootfs(const ExecContext *c) {
         return !empty_or_root(c->root_directory) || c->root_image || c->root_directory_as_fd;
 }
 
+bool exec_context_with_rootfs_strict(const ExecContext *c) {
+        assert(c);
+
+        /* just like exec_context_with_rootfs(), but doesn't suppress a root directory of "/", i.e. returns
+         * true in more cases: when a root directory is explicitly configured, even if it's our usual
+         * root. */
+
+        return c->root_directory || c->root_image || c->root_directory_as_fd;
+}
+
 int exec_context_has_vpicked_extensions(const ExecContext *context) {
         int r;