cmd/syncthing: Enable KCP by default

Also, use upstream library, as my changes have been merged.

vendor/github.com/templexxx/reedsolomon/rs_other.go

@@ -0,0 +1,8 @@
+// +build !amd64
+
+package reedsolomon
+
+func newRS(d, p int, em matrix) (enc Encoder) {
+	g := em[d*d:]
+	return &encBase{data: d, parity: p, encode: em, gen: g}
+}