New security feature: Redis protected mode.

An exposed Redis instance on the internet can be cause of serious
issues. Since Redis, by default, binds to all the interfaces, it is easy
to forget an instance without any protection layer, for error.

Protected mode try to address this feature in a soft way, providing a
layer of protection, but giving clues to Redis users about why the
server is not accepting connections.

When protected mode is enabeld (the default), and if there are no
minumum hints about the fact the server is properly configured (no
"bind" directive is used in order to restrict the server to certain
interfaces, nor a password is set), clients connecting from external
intefaces are refused with an error explaining what to do in order to
fix the issue.

Clients connecting from the IPv4 and IPv6 lookback interfaces are still
accepted normally, similarly Unix domain socket connections are not
restricted in any way.
