Log the name of the error as well as the error code if a network problem
happens. This makes network troubleshooting more convenient.

Use thread-safe strerror_r and the WIN32 equivalent FormatMessage.

The year is 2014. All supported operating systems have IPv6 support,
most certainly at build time (this doesn't mean that IPv6 is configured,
of course).

If noone is exercising the functionality to disable it, that means it
doesn't get tested, and IMO it's better to get rid of it.

(it's also not used consistently in RPC/boost and Net code...)

Use misc methods of avoiding unnecesary header includes.
Replace int typedefs with int##_t from stdint.h.
Replace PRI64[xdu] with PRI[xdu]64 from inttypes.h.
Normalize QT_VERSION ifs where possible.
Resolve some indirect dependencies as direct ones.
Remove extern declarations from .cpp files.

I2P apparently needs 256 bits to store a fully routable address. Garlicat
requires a centralized lookup service to map the 80-bit addresses to fully
routable ones (as far as I understood), so that's not really usable in our
situation.

To support I2P routing and peer exchange for it, another solution is needed.
This will most likely imply a network protocol change, and extension of the
'addr' message.

- fix #1560 by properly locking proxy related data-structures
- update GetProxy() and introduce GetNameProxy() to be able to use a
  thread-safe local copy from proxyInfo and nameproxyInfo
- update usage of GetProxy() all over the source to match the new
  behaviour, as it now fills a full proxyType object
- rename GetNameProxy() into HaveNameProxy() to be more clear

- I checked every occurance of strprintf() in the code and used %u, where
  unsigned vars are used
- the change to GetByte() was made, as ip is an unsigned char

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.

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.

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.

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.