net: introduce `Socket#connecting` property

There is no official way to figure out if the socket that you have on
hand is still connecting to the remote host. Introduce
`Socket#connecting`, which is essentially an unprefixed `_connecting`
property that we already had.

PR-URL: https://github.com/nodejs/node/pull/6404


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>