box/console: Provide console.eos() api

The other modules would be able to find out which eos marker
is currently active. For example when reading replies from
remote server via text based console protocol.

@TarantoolBot document
Title: > require('console').eos()

Returns a string with currently active end of string marker.

Part-of #3834

Reviewed-by: Konstantin Osipov <kostja.osipov@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>