Respect known_hosts file set by the user.

Thanks to contact@leblanc-simon.eu for the patch.

libssh/keyfiles.c

@@ -1501,10 +1501,12 @@ int ssh_write_knownhost(ssh_session session) {
   char *dir;
   size_t len = 0;
 
+  if (session->knownhosts == NULL) {
     if (ssh_options_set(session, SSH_OPTIONS_KNOWNHOSTS, NULL) < 0) {
-    ssh_set_error(session, SSH_FATAL, "Cannot find known_hosts file.");
+      ssh_set_error(session, SSH_FATAL, "Can't find a known_hosts file");
       return -1;
     }
+  }
 
   if (session->host == NULL) {
     ssh_set_error(session, SSH_FATAL,