apache-plugin: add Apache2 FastCGI plugin

This patch adds a test plugin for checkpointing Apache2 with
PHP-FastCGI.

Testing:
 - Build dmtcp
 - Build plugin
 - make check
 - Checkpoint-restart of Apache2 with PHP-FastCGI
   + Restart from a checkpoint taken before launching php
   + Restart from a checkpoint after launching php-fastcgi