refactor outgoing HTTP connection support

- style, cleanup, and removal of unnecessary code.
- combined isc_nm_http_add_endpoint() and isc_nm_http_add_doh_endpoint()
  into one function, renamed isc_http_endpoint().
- moved isc_nm_http_connect_send_request() into doh_test.c as a helper
  function; remove it from the public API.
- renamed isc_http2 and isc_nm_http2 types and functions to just isc_http
  and isc_nm_http, for consistency with other existing names.
- shortened a number of long names.
- the caller is now responsible for determining the peer address.
  in isc_nm_httpconnect(); this eliminates the need to parse the URI
  and the dependency on an external resolver.
- the caller is also now responsible for creating the SSL client context,
  for consistency with isc_nm_tlsdnsconnect().
- added setter functions for HTTP/2 ALPN. instead of setting up ALPN in
  isc_tlsctx_createclient(), we now have a function
  isc_tlsctx_enable_http2client_alpn() that can be run from
  isc_nm_httpconnect().
- refactored isc_nm_httprequest() into separate read and send functions.
  isc_nm_send() or isc_nm_read() is called on an http socket, it will
  be stored until a corresponding isc_nm_read() or _send() arrives; when
  we have both halves of the pair the HTTP request will be initiated.
- isc_nm_httprequest() is renamed isc__nm_http_request() for use as an
  internal helper function by the DoH unit test. (eventually doh_test
  should be rewritten to use read and send, and this function should
  be removed.)
- added implementations of isc__nm_tls_settimeout() and
  isc__nm_http_settimeout().
- increased NGHTTP2 header block length for client connections to 128K.
- use isc_mem_t for internal memory allocations inside nghttp2, to
  help track memory leaks.
- send "Cache-Control" header in requests and responses. (note:
  currently we try to bypass HTTP caching proxies, but ideally we should
  interact with them: https://tools.ietf.org/html/rfc8484#section-5.1)

lib/isc/netmgr/tcp.c

@@ -167,23 +167,6 @@ tcp_connect_direct(isc_nmsocket_t *sock, isc__nm_uvreq_t *req) {
 	REQUIRE(isc__nm_in_netthread());
 	REQUIRE(sock->tid == isc_nm_tid());
 
-	result = isc__nm_socket(req->peer.type.sa.sa_family, SOCK_STREAM, 0,
-				&sock->fd);
-	/*
-	 * The socket() call can fail spuriously on FreeBSD 12, so we need to
-	 * handle the failure early and gracefully.
-	 */
-	if (result != ISC_R_SUCCESS) {
-		atomic_store(&sock->closed, true);
-		isc__nm_uvreq_t *cbreq = NULL;
-		cbreq = isc__nm_uvreq_get(sock->mgr, sock);
-		cbreq->cb.connect = req->cb.connect;
-		cbreq->cbarg = req->cbarg;
-		isc_nmhandle_attach(req->handle, &cbreq->handle);
-		isc__nmsocket_clearcb(sock);
-		isc__nm_connectcb(sock, cbreq, result);
-		goto error;
-	}
 	result = isc__nm_socket_connectiontimeout(sock->fd,
 						  sock->connect_timeout);
 	RUNTIME_CHECK(result == ISC_R_SUCCESS);
@@ -231,7 +214,7 @@ tcp_connect_direct(isc_nmsocket_t *sock, isc__nm_uvreq_t *req) {
 
 done:
 	result = isc__nm_uverr2result(r);
-error:
+
 	LOCK(&sock->lock);
 	sock->result = result;
 	SIGNAL(&sock->cond);
@@ -260,7 +243,6 @@ isc__nm_async_tcpconnect(isc__networker_t *worker, isc__netievent_t *ev0) {
 	REQUIRE(sock->parent == NULL);
 	REQUIRE(sock->tid == isc_nm_tid());
 
-	sock->fd = (uv_os_sock_t)(-1);
 	result = tcp_connect_direct(sock, req);
 	if (result != ISC_R_SUCCESS) {
 		atomic_store(&sock->active, false);
@@ -333,17 +315,31 @@ isc_nm_tcpconnect(isc_nm_t *mgr, isc_nmiface_t *local, isc_nmiface_t *peer,
 	isc_nmsocket_t *sock = NULL;
 	isc__netievent_tcpconnect_t *ievent = NULL;
 	isc__nm_uvreq_t *req = NULL;
+	sa_family_t sa_family;
+	uv_os_sock_t fd;
 
 	REQUIRE(VALID_NM(mgr));
 	REQUIRE(local != NULL);
 	REQUIRE(peer != NULL);
 
+	sa_family = peer->addr.type.sa.sa_family;
+
+	/*
+	 * The socket() call can fail spuriously on FreeBSD 12, so we need to
+	 * handle the failure early and gracefully.
+	 */
+	result = isc__nm_socket(sa_family, SOCK_STREAM, 0, &fd);
+	if (result != ISC_R_SUCCESS) {
+		return (result);
+	}
+
 	sock = isc_mem_get(mgr->mctx, sizeof(*sock));
 	isc__nmsocket_init(sock, mgr, isc_nm_tcpsocket, local);
 
 	sock->extrahandlesize = extrahandlesize;
 	sock->connect_timeout = timeout;
 	sock->result = ISC_R_DEFAULT;
+	sock->fd = fd;
 	atomic_init(&sock->client, true);
 
 	req = isc__nm_uvreq_get(mgr, sock);