Windows: fix EFI DcsProp rewrite handling

Ensure ESP file writes have true replace semantics even when the operation is delegated to the elevated COM helper. This prevents shorter edits of EFI\VeraCrypt\DcsProp from leaving stale bytes at the end of the file.

Also XML-escape decoded EFI boot configuration values before serializing them, preserving values containing characters such as <, > and & during EfiBootConf save/update paths.

Fixes #954.

src/Common/BootEncryption.h

@@ -44,6 +44,7 @@ namespace VeraCrypt
 		DWORD Read (uint8 *buffer, DWORD size);
 		void Write (uint8 *buffer, DWORD size);
 		void SeekAt (int64 position);
+		void SetEnd ();
 		void GetFileSize (unsigned __int64& size);
 		void GetFileSize (DWORD& dwSize);
       bool IoCtl(DWORD code, void* inBuf, DWORD inBufSize, void* outBuf, DWORD outBufSize);