Major plugin refactor and cleanup.

Switched to POCO library for unified platform/library interface.
Deprecated the external module API. It was creating more problems than solving.
Removed most built-in libraries in favor of system libraries for easier maintenance.
Cleaned and secured code with help from static analyzers.

vendor/POCO/appveyor/Cygwin/excluded.sh

@@ -0,0 +1,2 @@
+export EXCLUDE_TESTS="Foundation Data/ODBC Data/MySQL Redis PDF"
+

vendor/POCO/appveyor/Cygwin/runtests.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+#
+# set -ev
+#
+set -v
+export POCO_BASE=`pwd`
+export PATH=$PATH:.
+export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:.
+source ./appveyor/ignored.sh
+source ./appveyor/Cygwin/excluded.sh
+build/script/runtests.sh