lib/protocol: Hello message length is an int16

It used to be an int32, but that's unnecessary and the spec now says
int16. Also relaxes the size requirement to that which fits in a signed
int16 instead of limiting to 1024 bytes, to allow for future growth.

As reported in
https://forum.syncthing.net/t/difference-between-documented-and-implemented-protocol/7798

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3406

lib/protocol/hello_test.go

@@ -26,9 +26,9 @@ func TestVersion14Hello(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	hdrBuf := make([]byte, 8)
+	hdrBuf := make([]byte, 6)
 	binary.BigEndian.PutUint32(hdrBuf, HelloMessageMagic)
-	binary.BigEndian.PutUint32(hdrBuf[4:], uint32(len(msgBuf)))
+	binary.BigEndian.PutUint16(hdrBuf[4:], uint16(len(msgBuf)))
 
 	outBuf := new(bytes.Buffer)
 	outBuf.Write(hdrBuf)