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/cmake/FindApache2.cmake

@@ -0,0 +1,49 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+#.rst:
+# FindApache2
+# -------
+#
+# Find Apache2 Runtime
+#
+# This will define the following variables::
+#
+#   APACHE2_FOUND           - True if the system has the Apache2 library
+#   APACHE2_INCLUDE_DIRS    - where to find httpd.h, etc.
+#
+# Hints:
+# Set ``APACHE2_ROOT_DIR`` to the root directory of an Apache installation.
+#
+include(FindPackageHandleStandardArgs)
+
+find_package(PkgConfig QUIET)
+pkg_check_modules(PC_APACHE2 QUIET apache2)
+
+find_path(APACHE2_INCLUDE_DIR httpd.h
+	HINTS
+		${APACHE2_ROOT_DIR}/include/apache2
+		${APACHE2_ROOT_INCLUDE_DIRS}
+	PATHS
+		${PC_APACHE2_INCLUDE_DIRS}
+		/usr/local/include/apache2
+		/usr/include/apache2
+)
+
+set(APACHE2_VERSION ${PC_APACHE2_VERSION})
+
+find_package_handle_standard_args(Apache2
+	FOUND_VAR APACHE2_FOUND
+	REQUIRED_VARS
+		APACHE2_INCLUDE_DIR
+	VERSION_VAR APACHE2_VERSION
+)
+
+if(APACHE2_FOUND)
+	set(APACHE2_INCLUDE_DIRS ${APACHE2_INCLUDE_DIR})
+	set(APACHE2_DEFINITIONS ${PC_APACHE2_CFLAGS_OTHER})
+endif()
+
+mark_as_advanced(
+	APACHE2_INCLUDE_DIR
+)