Add linter: Enforce the source code file naming convention described in the developer notes

test/lint/lint-filenames.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+#
+# Copyright (c) 2018 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# Make sure only lowercase alphanumerics (a-z0-9), underscores (_),
+# hyphens (-) and dots (.) are used in source code filenames.
+
+export LC_ALL=C
+
+EXIT_CODE=0
+OUTPUT=$(git ls-files -- "*.cpp" "*.h" "*.py" "*.sh" | grep -vE '^[a-z0-9_./-]+$' | grep -vE 'src/(secp256k1|univalue)/')
+if [[ ${OUTPUT} != "" ]]; then
+    echo "Use only lowercase alphanumerics (a-z0-9), underscores (_), hyphens (-) and dots (.)"
+    echo "in source code filenames:"
+    echo
+    echo "${OUTPUT}"
+    EXIT_CODE=1
+fi
+exit ${EXIT_CODE}