Fix Cygwin compilation:

- Fix various incompatibilities and differences between Linux and Cygwin builds
- Make usrsctp an optional dependency
- Fix building without SSL
- Add new secure random bytes function, use it for websockets
- Switch to libsrtp2 v2.6.0 (currently latest release)
- Add patch that makes latest libsrtp2 build in latest Cygwin
- Add patch that makes srt build in latest Cygwin
- Correctly allow linking libsrtp2 and srt to local mbedtls version

lib/config.cpp

@@ -927,7 +927,9 @@ void Util::getMyExec(std::deque<std::string> &execs){
   WIN32_FIND_DATA FindFileData;
   HANDLE hdl = FindFirstFile(path.c_str(), &FindFileData);
   while (hdl != INVALID_HANDLE_VALUE){
-    execs.push_back(FindFileData.cFileName);
+    if (!(FindFileData.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)){
+      execs.push_back(FindFileData.cFileName);
+    }
     if (!FindNextFile(hdl, &FindFileData)){
       FindClose(hdl);
       hdl = INVALID_HANDLE_VALUE;