move the socket into Rack env for hijacking
This avoids the expensive generic instance variable for @socket
and exposes the socket as `env["unicorn.socket"]' to the Rack
application.
As as nice side-effect, applications may access
`env["unicorn.socket"]' as part of the API may be useful for
3rd-party bits such as Raindrops::TCP_Info for reading the tcp_info
struct on Linux-based systems.
Yes, `env["unicorn.socket"]' is a proprietary API in unicorn!
News at 11! But then again, unicorn is not the first Rack server
to implement `env["#{servername}.socket"]', either...