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

subprojects/packagefiles/libsrtp_cygwin_patch

@@ -0,0 +1,15 @@
+diff --git a/meson.build b/meson.build
+index 81a232e..1f15de9 100644
+--- a/meson.build
++++ b/meson.build
+@@ -43,6 +43,10 @@ foreach h : check_headers
+   endif
+ endforeach
+
++if (host_system == 'cygwin')
++  cdata.set('HAVE_MACHINE_TYPES_H', false)
++endif
++
+ check_functions = [
+   'sigaction',
+   'inet_aton',