MEDIUM: mux-fcgi: Add the FCGI multiplexer

MEDIUM: mux-fcgi: Add the FCGI multiplexer

This multiplexer is only available on the backend side. It may handle
multiplexed connections if the FCGI application supports it. A FCGI application
must be configured on the backend to be used. If not redefined during the
request processing by the FCGI filter, this mux handles all mandatory
parameters.

There is a limitation on the way the requests are processed. The parameters must
be encoded into a uniq PARAMS record. It means, once encoded, all HTTP headers
and FCGI parameters must small enough to be store in a buffer. Otherwise, an
internal processing error is returned.