Wip rgw standalone 2

Add an experimental standalone embedded web server. We use mongoose, and it is possible to define multiple frontends on a single gateway. The configurable 'rgw frontends' can be set to define which frontends to use (and for mongoose, on which port), e.g.,

rgw frontend = fastcgi, mongoose:8080, mongoose:8081

We might also make it so that we could have multiple fastcgi sockets configured (e.g., fastcgi:/var/run/radosgw/rgw.sock,fastcgi:host:9000), however, at the moment the fastcgi configuration hasn't changed.