libevent uses getaddrinfo when available, and falls back to gethostbyname
Windows has both, but gethostbyname only supports IPv4
libevent fails to detect Windows's getaddrinfo due to not including the right headers
This patches libevent's configure script to check it correctly

Github-Pull: bitcoin#18287
Rebased-From: e918138

…ipv6 usage

eb6b735 build: pass _WIN32_WINNT=0x0601 when building libevent for Windows (fanquake)
03e056e depends: Patch libevent build to fix IPv6 -rpcbind on Windows (Luke Dashjr)

Pull request description:

  TLDR: This poaches a commit from #18287 and adds one more to adjust the Windows version targeted when building libevent. These changes combined should fully fix ipv6 usage with the RPC server on Windows.

  ---

  Binding the RPC server to a ipv6 address does not currently work on Windows.
  We currently try and bind to `127.0.0.1` and `::1` [by default](https://github.com/bitcoin/bitcoin/blob/master/src/httpserver.cpp#L304).

  On Windows you'll see lines like this in debug.log:
  ```bash
  2020-06-24T01:49:04Z libevent: getaddrinfo: nodename nor servname provided, or not known
  2020-06-24T01:49:04Z Binding RPC on address ::1 port 8332 failed
  ```

  This issue was bought up in, and supposedly fixed by #18287, however the two people that tested it, both said that it didn't fix the problem. I think I now understand why that change alone is incomplete.

  Our call into libevent starts with [evhttp_bind_socket_with_handle()](https://github.com/bitcoin/bitcoin/blob/master/src/httpserver.cpp#L325):
  ```bash
  evhttp_bind_socket_with_handle()
  	bind_socket()
  		make_addrinfo()
  			evutil_getaddrinfo()
  				if #USE_NATIVE_GETADDRINFO
  					#ifndef AI_ADDRCONFIG
  						evutil_adjust_hints_for_addrconfig_()
  							evutil_check_interfaces()
  								evutil_check_ifaddrs()
  									evutil_found_ifaddr()
  										// miss identifies ipv6 as ipv4?
  					#endif
  					evutil_getaddrinfo_common_()
  ```

  The problem is falling into ["#ifndef AI_ADDRCONFIG"](https://github.com/libevent/libevent/blob/master/evutil.c#L1580):

  ```cpp
  #ifndef AI_ADDRCONFIG
  	/* Not every system has AI_ADDRCONFIG, so fake it. */
  	if (hints.ai_family == PF_UNSPEC &&
  	    (hints.ai_flags & EVUTIL_AI_ADDRCONFIG)) {
  		evutil_adjust_hints_for_addrconfig_(&hints);
  	}
  #endif
  ```

  When this occurs, hints end up being adjusted, and it seems that ipv6 addresses end up being mis-identified as ipv4?

  However this shouldn't happen, as these `AI_` definitions are available on Windows.
  The issue is that in evutil.c, `_WIN32_WINNT` [is set to `0x501`](https://github.com/libevent/libevent/blob/master/evutil.c#L45) (XP).

  This obviously predates Vista (`0x0600`), which is when the `AI_ADDRCONFIG` definition (and others) became [available](https://docs.microsoft.com/en-us/windows/win32/api/ws2def/ns-ws2def-addrinfoa).

  The change here will override libevents internal D_WIN32_WINNT defines. This should be ok, because it's only making "more" of the Windows API available. It's also aligned with what we do in our own configure, we pass [`D_WIN32_WINNT=0x0601`](https://github.com/bitcoin/bitcoin/blob/master/configure.ac#L610). We also now use linker flags to restrict our binary from running on a Windows version [earlier than Windows 7](https://github.com/bitcoin/bitcoin/blob/master/configure.ac#L621).

  The combined fixes can be tested by running:

  `bitcoind -rpcbind=::1 rpcallowip='0.0.0.0/0' -debug=http`

  and then querying it using:

  `bitcoin-cli -rpcconnect=::1 getblockchaininfo`

  TODO:
  - [x] Open an issue upstream. libevent/libevent#1041

ACKs for top commit:
  laanwj:
    ACK eb6b735

Tree-SHA512: e1e50f194911301981edaed0c216ed4efb9ebd4a1f9bc9b9f85bec7140b66c45c8666fd5db4aad359596559d4a08ab7c920e9d9736f3ecdbb841afc54e40586e