Fix Incorrect Directory Replacement

On some installs, the directory ends up being /usr/pbi/dansguardian-xxx/...
This results in include lines not being written correctly and broken config files. DANSGUARDIAN_DIR should be present on every system.
Contentscanners preg_replace

This condition also applies to the contentscanner section, and there is no variable substitution going on. Does not affect installs that use /usr/local, and allows the correct behavior on installs in /usr/pbi/dansguardian/... or wherever else (as long as DANSGUARDIAN_DIR is the correct directory).
Merge pull request #474 from timdufrane/master

Fix Incorrect Directory Replacement