few optimizations building string vector with arguments

- avoid unneeded copying (don't gather remaining string)
- don't build args byte by byte

Signed-off-by: Sergey G. Brester <serg.brester@sebres.de>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>

CPP/Common/MyString.h

@@ -628,6 +628,7 @@ public:
   UString &operator=(char c) { return (*this)=((wchar_t)(unsigned char)c); }
   UString &operator=(const wchar_t *s);
   UString &operator=(const UString &s);
+  void AddFrom(const wchar_t *s, unsigned len); // no check
   void SetFrom(const wchar_t *s, unsigned len); // no check
   void SetFromBstr(LPCOLESTR s);
   UString &operator=(const char *s);