[darwin] Add function to get iOS version as a string
[darwin] Add function for getting OSX version string
SysInfo::GetUserAgent: fix UserAgent for iOS and OSX
SysInfo: some removal of CStdString
[android] SysInfo: add GetAndroidVersionString() function
[android] SysInfo: add GetAndroidDeviceName() function
SysInfo::GetUserAgent: fix Android UserAgent string