Add a lint check for trailing whitespace.

This adds a new CHECK_DOC check that looks for newly introduced trailing
whitespace. Existing trailing whitespace (of which there is plenty!)
will not trigger an error.

This is written in a generic way so that new lint-*.sh scripts can be
added to contrib/devtools/, as I'd like to contribute additional lint
checks in the future.

contrib/devtools/lint-all.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+#
+# Copyright (c) 2017 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# This script runs all contrib/devtools/lint-*.sh files, and fails if any exit
+# with a non-zero status code.
+
+set -u
+
+SCRIPTDIR=$(dirname "${BASH_SOURCE[0]}")
+LINTALL=$(basename "${BASH_SOURCE[0]}")
+
+for f in "${SCRIPTDIR}"/lint-*.sh; do
+  if [ "$(basename "$f")" != "$LINTALL" ]; then
+    if ! "$f"; then
+      echo "^---- failure generated from $f"
+      exit 1
+    fi
+  fi
+done