core: Try to prevent infinite recursive template instantiation

To prevent situations like in #17602 from happening, let's drop
direct recursive template dependencies. These will almost certainly
lead to infinite recursion so let's drop them immediately to avoid
instantiating potentially thousands of irrelevant units.

Example of a template that would lead to infinite recursion which
is caught by this check:

notify@.service:

```
[Unit]
Wants=notify@%n.service
```

src/basic/unit-name.c

@@ -8,6 +8,7 @@
 #include "alloc-util.h"
 #include "glob-util.h"
 #include "hexdecoct.h"
+#include "memory-util.h"
 #include "path-util.h"
 #include "special.h"
 #include "string-util.h"
@@ -797,3 +798,26 @@ bool slice_name_is_valid(const char *name) {
 
         return true;
 }
+
+bool unit_name_prefix_equal(const char *a, const char *b) {
+        const char *p, *q;
+
+        assert(a);
+        assert(b);
+
+        if (!unit_name_is_valid(a, UNIT_NAME_ANY) || !unit_name_is_valid(b, UNIT_NAME_ANY))
+                return false;
+
+        p = strchr(a, '@');
+        if (!p)
+                p = strrchr(a, '.');
+
+        q = strchr(b, '@');
+        if (!q)
+                q = strrchr(b, '.');
+
+        assert(p);
+        assert(q);
+
+        return memcmp_nn(a, p - a, b, q - b) == 0;
+}