Native IPv6 support for iperf

This simplifies the internal IPv6 concepts so that IPv4/IPv6 and IPv4/IPv6
hostnames can be used interchangeably, without having to use the -V flag.

That is, the type of the address (v4/v6) is automatically detected from the
input hostname given, as one would expect.

Most of the code is now Linux-specific, which is why #ifdefs have been removed.

Note on multicast-join operation for v4/v6:
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The multicast-join code has been extended to allow specifying the multicast
interface. This is clear when a host has only one interface: but on which
interface to join if the host has more than one interface?
This is now done via the
	-J <interface-name>  or
	-j <interface-name> flags.

Note on bidirectional/tradeoff tests (options -2 / -r):
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The connecting client does not longer validate the peer address when switching
from a connecting client to a listening server for the reverse connection. The
reason is that previously mSettings->peer was looked up and then tested against
the server->peer address. When using both v4/v6 addresses, this is no longer
trivial (v4/v6, v4/v4, v6/v4, v6/v6), hence I have disabled it.