apache-plugin: add Apache2 FastCGI plugin

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

Refer to https://github.com/dmtcp/dmtcp/pull/5 for the review
comments.

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



git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2861 ba6dc0ae-232f-0410-883f-9ebac64fc565