Add convenience ntstring::operator=(const char (&)[N])

OS X-specific configuration code assumes the ability to assign a literal
string to an ntstring.  Add an operator= overload to support that.

Reported by kreatordxx: https://github.com/dxx-rebirth/dxx-rebirth/issues/30

common/include/ntstring.h

@@ -84,4 +84,11 @@ public:
 	{
 		return !(*this == r);
 	}
+	template <std::size_t N>
+		ntstring &operator=(const char (&i)[N])
+		{
+			static_assert(N <= L, "string too long");
+			copy_if(i);
+			return *this;
+		}
 };