Improve Lwan detection by passing native_request to remy.detect()

Lwan detection is quite hackish at the moment (not unlike the other
ones, though), but it is good enough and still better than presuming
Remy is running under Lwan if all other modes fail for some reason.
Avoid autodetecting environment each and every request

This caches the result of remy.detect() inside the sailor table, and
reuses it next time remy.init() is called.  This works since the
`remy_mode` is `nil` the first time, which is a synonym for
`remy.MODE_AUTODETECT`; it is then assigned to whatever value defines
the current environment.