Add support for Tor/I2P networks, and make code more readable.

This commit adds support for .onion addresses (mapped into the IPv6
by using OnionCat's range and encoding), and the ability to connect
to them via a SOCKS5 proxy.

* Do not rely on "a.b.c" being interpreted as "a.0.b.c"
* Parse numeric addresses for address families for which no device is configured

Implement the following rules:
* Interpret [X]:Y as host=X port=Y, if Y is an integer
* Interpret X:Y as host=X port=Y, if Y is an integer and X contains no colon
* Interpret X:Y as host=X:Y port=default otherwise

I originally created a pull to replace the "COPYING" in crypter.cpp and
crypter.h, but it turned out that COPYING was actually the correct
file.

This prevents an undefined operation in main.cpp, when shifting the hash value
left by 32 bits.
Shifting a signed int left into the sign bit is undefined in C++11.

use "IPv6" and "IPv4" in strings as these are the official spellings and make ParseNetwork() in netbase.cpp case-insensitive

Introduce a boolean variable for each "network" (ipv4, ipv6, tor, i2p),
and track whether we are likely to able to connect to it. Addresses in
"addr" messages outside of our network get limited relaying and are not
stored in addrman.

There are plans to let Bitcoin function as Tor/I2P hidden service.
To do so, we could use the established encoding provided by OnionCat
and GarliCat (without actually using those tools) to embed Tor/I2P
addresses in IPv6.

This patch makes these addresses considered routable, so they can
travel over the Bitcoin network in 'addr' messages. This will hopefully
make it easier to deploy real hidden service support later.

This will make bitcoin relay valid routable IPv6 addresses, and when
USE_IPV6 is enabled, listen on IPv6 interfaces and attempt connections
to IPv6 addresses.

* Use -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameters
* Remove xCXXFLAGS usage in makefile.unix
* Fix several recent and older sign-compare warnings

Add -socks=<n> to select SOCKS version to use. 4 and 5 are supported,
5 is default.

foo.size() typically returns an unsigned integral type; make loop variables
match those types' signedness.

Design goals:
 * Only keep a limited number of addresses around, so that addr.dat does not grow without bound.
 * Keep the address tables in-memory, and occasionally write the table to addr.dat.
 * Make sure no (localized) attacker can fill the entire table with his nodes/addresses.

See comments in addrman.h for more detailed information.

This turns on most gcc warnings, and removes some unused variables and other code that triggers warnings.
Exceptions are:
 -Wno-sign-compare : triggered by lots of comparisons of signed integer to foo.size(), which is unsigned.
 -Wno-char-subscripts : triggered by the convert-to-hex functions (I may fix this in a future commit).

This introduces CNetAddr and CService, respectively wrapping an
(IPv6) IP address and an IP+port combination. This functionality used
to be part of CAddress, which also contains network flags and
connection attempt information. These extra fields are however not
always necessary.

These classes, along with logic for creating connections and doing
name lookups, are moved to netbase.{h,cpp}, which does not depend on
headers.h.

Furthermore, CNetAddr is mostly IPv6-ready, though IPv6
functionality is not yet enabled for the application itself.