protect against cancellation in dlopen

i'm not sure that it's "correct" for dlopen to block cancellation
when calling constructors for libraries it loads, but it sure seems
like the right thing. in any case, dlopen itself needs cancellation
blocked.

src/ldso/dynlink.c

@@ -610,9 +610,11 @@ void *dlopen(const char *file, int mode)
 {
 	struct dso *volatile p, *orig_tail = tail, *next;
 	size_t i;
+	int cs;
 
 	if (!file) return head;
 
+	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
 	pthread_rwlock_wrlock(&lock);
 
 	if (setjmp(rtld_fail)) {
@@ -628,8 +630,8 @@ void *dlopen(const char *file, int mode)
 		}
 		tail = orig_tail;
 		tail->next = 0;
-		pthread_rwlock_unlock(&lock);
-		return 0;
+		p = 0;
+		goto end;
 	}
 
 	p = load_library(file);
@@ -658,6 +660,7 @@ void *dlopen(const char *file, int mode)
 	do_init_fini(tail);
 end:
 	pthread_rwlock_unlock(&lock);
+	pthread_setcancelstate(cs, 0);
 	return p;
 }