Common: Add Random utilities

This makes it easier to generate random numbers or fill a buffer with
random data in a cryptographically secure way.

This also replaces existing usages of RNG functions in the codebase:

* <random> is pretty hard to use correctly, and std::random_device does
  not give enough guarantees about its results (it's
  implementation-defined, non cryptographically secure and could be
  deterministic on some platforms).
  Doing things correctly is error prone and verbose.

* rand() is terrible and should not be used especially in crypto code.

Source/Core/Common/CMakeLists.txt

@@ -34,6 +34,7 @@ add_library(common
   PerformanceCounter.cpp
   Profiler.cpp
   QoSSession.cpp
+  Random.cpp
   SDCardUtil.cpp
   SettingsHandler.cpp
   StringUtil.cpp
@@ -156,6 +157,7 @@ endif()
 if(UNIX)
   # Posix networking code needs to be fixed for Windows
   add_executable(traversal_server TraversalServer.cpp)
+  target_link_libraries(traversal_server PRIVATE common)
   if(SYSTEMD_FOUND)
     target_link_libraries(traversal_server ${SYSTEMD_LIBRARIES})
   endif()