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.cpp

@@ -1206,6 +1206,16 @@ UString &UString::operator=(const UString &s)
   return *this;
 }
 
+void UString::AddFrom(const wchar_t *s, unsigned len) // no check
+{
+  if (len) {
+    Grow(len);
+    wmemcpy(_chars + _len, s, len);
+    _len += len;
+    _chars[_len] = 0;
+  }
+}
+
 void UString::SetFrom(const wchar_t *s, unsigned len) // no check
 {
   if (len > _limit)