qpkg: some cleanup, prerequisite search speedup

With the reduction of search depth, "prereq abiword" takes about 3 minutes
on my PC. (Obviously, this can still be improved.)

- gobble.c (gobble_buf): if pkg->id->value is NULL, just it as such
- prereq.c (push): abort if trying to consider a package already considered
  or installed
- prereq.c (conflicts): before being satisfied with using an installed
  package, make sure it really satisfies the requirement
- prereq.c (conflicts): abort if an installed package conflicts
- prereq.c (conflicts): added some debugging output (temporary)
- prereq.c (conflicts): cut the search if we can't do better than a
  previous match

qpkg/gobble.c

@@ -241,7 +241,7 @@ package:
 	WHITESPACE;
 	pkg = alloc_type(struct pkg);
 	pkg->id = ID(packages);
-	pkg->more = pkg->id->value ? pkg->id->value : NULL;
+	pkg->more = pkg->id->value;
 	pkg->id->value = pkg;
 	pkg->version = NULL;
 	pkg->filename = NULL;

qpkg/prereq.c

@@ -111,6 +111,7 @@ static void push(struct pkg *pkg)
 		
 	}
 	stack[n_stack++] = pkg;
+	assert(!pkg->mark && !pkg->installed);
 	pkg->mark = 1;
 }
 
@@ -158,6 +159,7 @@ static int conflicts(const struct pkg *pkg, const struct list *conf)
 static void resolve(struct list *next_dep, const struct ref *dep,
     struct list *conf)
 {
+static int level = 0;
 	struct list more_deps;
 	struct list more_conf = {
 		.next = conf
@@ -173,14 +175,27 @@ static void resolve(struct list *next_dep, const struct ref *dep,
 		next_dep = next_dep->next;
 	}
 	for (pkg = dep->pkg->value; pkg; pkg = pkg->more) {
+		if (best && n_stack == n_best)
+			return;
+#if 0
+fprintf(stderr, "%*s", level, "");
+fprintf(stderr, "%.*s %p", ID2S(pkg->id), pkg);
+if (pkg->version)
+fprintf(stderr, " %.*s", ID2S(pkg->version));
+if (pkg->mark) fprintf(stderr, " +");
+if (pkg->installed) fprintf(stderr, " ***");
+fprintf(stderr, "\n");
+#endif
+		if (!satisfies(pkg, dep))
+			continue;
 		if (pkg->installed || pkg->mark) {
+			assert(!conflicts(pkg, conf));
 			resolve(next_dep, dep->next, conf);
 			continue;
 		}
-		if (!satisfies(pkg, dep))
-			continue;
 		if (conflicts(pkg, conf))
 			continue;
+level++;
 		push(pkg);
 		more_deps.refs = pkg->depends;
 		more_deps.next = next_dep;
@@ -189,6 +204,7 @@ static void resolve(struct list *next_dep, const struct ref *dep,
 		resolve(&more_deps, dep->next, &more_conf);
 		next_dep = more_deps.next;
 		pop();
+level--;
 	}
 }