Add DirectX SDK (D3DX9) download support

download --with-dxsdk fetches the DirectX SDK (June 2010) - the last
standalone release of D3DX9/10/11, XInput and XAudio2, dropped from
the Windows SDK entirely once D3DX was deprecated, but still needed
by plenty of legacy code. Like the WDK, it isn't part of the VS
installer manifest, so this is its own self-contained fetch+unpack
path: the installer is a self-extracting CAB, unpacked directly via
cabextract (already a prerequisite for the WinSDK .msi payloads) with
its -F filter restricting extraction to just Include/ and Lib/ -
about 21MB out of the installer's 1.2GB uncompressed payload.

Verified against the real installer: real d3dx9.h and d3dx9.lib
(x86 and x64) extracted correctly and linked into an actual legacy
game client build.

internal/i18n/i18n.go

@@ -77,8 +77,8 @@ Usage:
   vintner completion bash|zsh          print a shell completion script
 
 Run "vintner <command> --help" for that command's own options - download
-has many, including --with-wdk, --list-workloads, --list-components and
---print-deps-tree.
+has many, including --with-wdk, --with-dxsdk, --list-workloads,
+--list-components and --print-deps-tree.
 
 --dest/[dir] default to ~/.vintner if omitted.
 Language: set VINTNER_LANG=ru (or LANG=ru_RU...) for Russian output.
@@ -99,8 +99,8 @@ Once installed, add <dir>/bin/<arch> to PATH and invoke the tools directly:
   vintner completion bash|zsh          вывести скрипт автодополнения для оболочки
 
 Запустите «vintner <команда> --help» для параметров конкретной команды —
-у download их много, включая --with-wdk, --list-workloads, --list-components
-и --print-deps-tree.
+у download их много, включая --with-wdk, --with-dxsdk, --list-workloads,
+--list-components и --print-deps-tree.
 
 --dest/[каталог] по умолчанию — ~/.vintner.
 Язык: установите VINTNER_LANG=en (или LANG=en_US...) для вывода на английском.
@@ -161,6 +161,10 @@ Once installed, add <dir>/bin/<arch> to PATH and invoke the tools directly:
 		EN: "Installed WDK (%s) %s at %s\n",
 		RU: "WDK (%s) %s установлен в %s\n",
 	},
+	"download.dxsdk_installed": {
+		EN: "Installed DirectX SDK (June 2010) at %s\n",
+		RU: "DirectX SDK (июнь 2010) установлен в %s\n",
+	},
 	"download.workloads_header": {
 		EN: "Available Workloads (%d):\n",
 		RU: "Доступные рабочие нагрузки (Workload) (%d):\n",