Make Lck_AssertHeld trigger in the caller's code

The `held == 0` branch in the Lck__AssertHeld function is *never*
taken and `cache.h` instructs to only use the macro. Being generated
from calling code, the panic message becomes more informative.

Panic messages were up until now not very helpful:

    Assert error in Lck__Assert(), cache/cache_lck.c line 175:
    Condition(ilck->held) not true.

bin/varnishd/cache/cache.h

@@ -840,7 +840,8 @@ void Lck__Lock(struct lock *lck, const char *p,  int l);
 void Lck__Unlock(struct lock *lck, const char *p,  int l);
 int Lck__Trylock(struct lock *lck, const char *p,  int l);
 void Lck__New(struct lock *lck, struct VSC_C_lck *, const char *);
-void Lck__Assert(const struct lock *lck, int held);
+int Lck__Held(const struct lock *lck);
+int Lck__Owned(const struct lock *lck);
 
 /* public interface: */
 void Lck_Delete(struct lock *lck);
@@ -850,7 +851,11 @@ int Lck_CondWait(pthread_cond_t *cond, struct lock *lck, double);
 #define Lck_Lock(a) Lck__Lock(a, __func__, __LINE__)
 #define Lck_Unlock(a) Lck__Unlock(a, __func__, __LINE__)
 #define Lck_Trylock(a) Lck__Trylock(a, __func__, __LINE__)
-#define Lck_AssertHeld(a) Lck__Assert(a, 1)
+#define Lck_AssertHeld(a) 		\
+	do {				\
+		assert(Lck__Held(a));	\
+		assert(Lck__Owned(a));	\
+	} while (0)
 
 struct VSC_C_lck *Lck_CreateClass(const char *name);
 

bin/varnishd/cache/cache_lck.c

@@ -165,19 +165,23 @@ Lck__Trylock(struct lock *lck, const char *p, int l)
 	return (r);
 }
 
-void
-Lck__Assert(const struct lock *lck, int held)
+int
+Lck__Held(const struct lock *lck)
 {
 	struct ilck *ilck;
 
 	CAST_OBJ_NOTNULL(ilck, lck->priv, ILCK_MAGIC);
-	if (held) {
-		assert(ilck->held);
-		assert(pthread_equal(ilck->owner, pthread_self()));
-	} else {
-		AZ(ilck->held);
-		AZ(pthread_equal(ilck->owner, pthread_self()));
-	}
+	return (ilck->held);
+}
+
+int
+Lck__Owned(const struct lock *lck)
+{
+	struct ilck *ilck;
+
+	CAST_OBJ_NOTNULL(ilck, lck->priv, ILCK_MAGIC);
+	AN(ilck->held);
+	return (pthread_equal(ilck->owner, pthread_self()));
 }
 
 int __match_proto__()