Add UTF-8 flag to the Windows message_box() wrapper.

message_box() previously differed from the real MessageBox API
function in that it permitted the user to provide a help context to be
used for a Help button in the dialog box.

Now it adds a second unusual ability: you can specify that the text
and caption strings are in UTF-8 rather than the system code page.

windows/utils/pgp_fingerprints_msgbox.c

@@ -21,5 +21,5 @@ void pgp_fingerprints_msgbox(HWND owner)
         ", " PGP_PREV_MASTER_KEY_DETAILS "):\n"
         "  " PGP_PREV_MASTER_KEY_FP,
         "PGP fingerprints", MB_ICONINFORMATION | MB_OK,
-        HELPCTXID(pgp_fingerprints));
+        false, HELPCTXID(pgp_fingerprints));
 }