CLEANUP: thread: rename __decl_hathreads() to __decl_thread()

I can never figure whether it takes an "s" or not, and in the end it's
better if it matches the file's naming, so let's call it "__decl_thread".
diff --git a/src/listener.c b/src/listener.c
index c035ed8..3315d60 100644
--- a/src/listener.c
+++ b/src/listener.c
@@ -735,7 +735,7 @@
 		struct sockaddr_storage addr;
 		socklen_t laddr = sizeof(addr);
 		unsigned int count;
-		__decl_hathreads(unsigned long mask);
+		__decl_thread(unsigned long mask);
 
 		/* pre-increase the number of connections without going too far.
 		 * We process the listener, then the proxy, then the process.