Refactor rt-server.in into RT::PlackRunner

This properly detects --port, --listen, and --socket options, and falls
back to $WebPort only when none are specified.  It also provides better
handling for fastcgi, detecting when STDIN is a socket, and bailing with
a correct error message if it has no other options to listen on.
Catch and fix var/ when rt-server is incorrectly run as root

rt-server often gets incorrectly run as root for testing, leaving the
mason cache later un-writable by the real web user (when run as fastcgi,
for example).  Attempt to correct for this by chown'ing the var path to
the web user and group if we were run as root.
Bump Plack dependency to the minimum which added default_middleware
Merge branch '4.2/refactor-rt-server'