MEDIUM: fcgi-app: Add FCGI application and filter

MEDIUM: fcgi-app: Add FCGI application and filter

The FCGI application handles all the configuration parameters used to format
requests sent to an application. The configuration of an application is grouped
in a dedicated section (fcgi-app <name>) and referenced in a backend to be used
(use-fcgi-app <name>). To be valid, a FCGI application must at least define a
document root. But it is also possible to set the default index, a regex to
split the script name and the path-info from the request URI, parameters to set
or unset...  In addition, this patch also adds a FCGI filter, responsible for
all processing on a stream.