Implement dial, support IPv6 in httpclient (#5763)

* Implement dial, support IPv6 in httpclient

Added ``dial`` procedure to networking modules: ``net``, ``asyncdispatch``,
``asyncnet``. It merges socket creation, address resolution, and connection
into single step. When using ``dial``, you don't have to worry about
IPv4 vs IPv6 problem.

Fixed addrInfo loop in connect to behave properly.
Previously it would stop on first non-immediate failure, instead of
continuing and trying the remaining addresses.

Fixed newAsyncNativeSocket to raise proper error if socket creation
fails.

Fixes: #3811

* Check domain during connect() only on non-Windows

This is how it was in the previous implementation of connect().

* Call 'osLastError' before 'close' in net.dial

* Record osLastError before freeAddrInfo in net.dial

* Add missing docs for 'dial' proc

* Optimize dial to create one FD per domain, add tests

And make async IPv6 servers work on Windows.

* Add IPv6 test to uri module

* Fix getAddrString error handling

lib/pure/asyncnet.nim

@@ -244,6 +244,17 @@ when defineSsl:
           else:
             raiseSSLError("Socket has been disconnected")
 
+proc dial*(address: string, port: Port, protocol = IPPROTO_TCP,
+           buffered = true): Future[AsyncSocket] {.async.} =
+  ## Establishes connection to the specified ``address``:``port`` pair via the
+  ## specified protocol. The procedure iterates through possible
+  ## resolutions of the ``address`` until it succeeds, meaning that it
+  ## seamlessly works with both IPv4 and IPv6.
+  ## Returns AsyncSocket ready to send or receive data.
+  let asyncFd = await asyncdispatch.dial(address, port, protocol)
+  let sockType = protocol.toSockType()
+  let domain = getSockDomain(asyncFd.SocketHandle)
+  result = newAsyncSocket(asyncFd, domain, sockType, protocol, buffered)
 
 proc connect*(socket: AsyncSocket, address: string, port: Port) {.async.} =
   ## Connects ``socket`` to server at ``address:port``.