Solution: introduced constant

Problem: inconsistent naming style for private data members, conflicts with naming of local variables and member functions

Solution: apply and check _lower_case naming style for private data members

Solution: remove else

Solution: define and apply parameter naming style: lower_case_

Solution: replace by C++-style casts

Solution: fix type specification

Solution: use proper types

Solution: Add IPv6 support

for multicast

Solution: augment the UDP URL syntax to accept an interface specifier with a
syntax similar to the PGM urls.

Fixes #2212

Solution: factor the code into ip_resolver, add IPv6 support and unit tests.

Solution: return an error in this situation but still allow using an explicit
"0" if somebody really wants to connect to port 0.

This shouldn't break any existing code because a "*" port was already rejected
in an early test in the TCP path in zmq::socket_base_t::connect.

Solution: Factor the code into a different file with a well defined API and add
unit tests.