修复未开启sctp时，webrtc datachannel sdp交换相关bug (#2791)

主要优化如下：
1、通过设置port为0 声明不支持datachannel(而不是inactive)。
2、设置正确的ice用户名和密码，防止ice建联失败。
3、group attribute移除datachannel的mid。
3、datachannel 相关sdp移除rtcp相关描述。

webrtc/Sdp.cpp

@@ -1555,7 +1555,10 @@ shared_ptr<RtcSession> RtcConfigure::createAnswer(const RtcSession &offer) const
     //设置音视频端口复用
     if (!offer.group.mids.empty()) {
         for (auto &m : ret->media) {
-            ret->group.mids.emplace_back(m.mid);
+            //The remote end has rejected (port 0) the m-section, so it should not be putting its mid in the group attribute.
+            if (m.port) {
+                ret->group.mids.emplace_back(m.mid);
+            }
         }
     }
     return ret;
@@ -1613,15 +1616,15 @@ RETRY:
     if (offer_media.type == TrackApplication) {
         RtcMedia answer_media = offer_media;
         answer_media.role = mathDtlsRole(offer_media.role);
-#ifdef ENABLE_SCTP
-        answer_media.direction = matchDirection(offer_media.direction, configure.direction);
-        answer_media.candidate = configure.candidate;
         answer_media.ice_ufrag = configure.ice_ufrag;
         answer_media.ice_pwd = configure.ice_pwd;
         answer_media.fingerprint = configure.fingerprint;
         answer_media.ice_lite = configure.ice_lite;
+#ifdef ENABLE_SCTP
+        answer_media.candidate = configure.candidate;
 #else
-        answer_media.direction = RtpDirection::inactive;
+        answer_media.port = 0;
+        WarnL << "answer sdp忽略application mline, 请安装usrsctp后再测试datachannel功能";
 #endif
         ret->media.emplace_back(answer_media);
         return;