Change sensitive strbufs/sgrowarrays to the new _nm version.

The _nm strategy is slower, so I don't want to just change everything
over no matter what its contents. In this pass I've tried to catch
everything that holds the _really_ sensitive things like passwords,
private keys and session keys.

sesschan.c

@@ -384,7 +384,7 @@ bool sesschan_enable_x11_forwarding(
      */
     if (authdata_hex.len % 2)
         return false;                  /* expected an even number of digits */
-    authdata_bin = strbuf_new();
+    authdata_bin = strbuf_new_nm();
     for (i = 0; i < authdata_hex.len; i += 2) {
         const unsigned char *hex = authdata_hex.ptr;
         char hexbuf[3];