[darwin] add getIosPlatformString(), refactoring getIosPlatform()
[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
SysInfo::GetUserAgent: fix Linux and FreeBSD UserAgent strings
[RPI] SysInfo::GetUserAgent: add unique identification of RPI without breaking Useragent parsing by web tools
SysInfo::GetUserAgent: add hardware identification for iOS devices
SysInfo::GetUserAgent: add placeholders for XBMC fork ID and device model ID
SysInfo::GetUserAgent: cache result