Box.cfg{listen = 0} automatically chooses a port. But it was
impossible to obtain a real port the instance is bound to.

An ability to see a real port may help to make test-run more
robust, because it won't depend on which ports are free, and
won't need to pre-choose them in advance.

Now box.info.listen shows a real address, or nil when listen is
turned off. Also a real address is logged instead of the dummy
0-port one.

Closes #4620

@TarantoolBot document
Title: box.info.listen - real address

New value in box.info - listen. It is a real address to which the
instance was bound. For example, if box.cfg.listen was set with
a zero port, box.info.listen will show a real port. The address
is stored as a string:

    - unix/:<path> for UNIX domain sockets;
    - <ip>:<port> for IPv4;
    - [ip]:<port> for IPv6.

If the instance does not listen anything, box.info.listen is nil.