Add generic mechanism to cross-ref settings at load time.

This is useful for loading settings from legacy/special versions,
where the corresponding setting is somewhere else and/or a PATX setting,
with duplicating all the info.

(cherry picked from commit b5c453b21e)

src/table/settings.ini

@@ -77,6 +77,7 @@ SDT_OMANY  =  SDT_OMANY($base, $var, $type, $flags, $guiflags, $def,
 SDT_STR    =    SDT_STR($base, $var, $type, $flags, $guiflags, $def,                              $str, $strhelp, $strval, $proc, $from, $to,        $cat, $extver, $patxname),
 SDT_VAR    =    SDT_VAR($base, $var, $type, $flags, $guiflags, $def,       $min, $max, $interval, $str, $strhelp, $strval, $proc, $from, $to,        $cat, $extver, $patxname),
 SDT_NULL   =   SDT_NULL($length, $from, $to, $extver),
+SDT_XREF     = SDT_XREF(         $from, $to, $extver, $xref),
 SDT_END    = SDT_END()
 
 [defaults]
@@ -93,6 +94,7 @@ to       = SL_MAX_VERSION
 cat      = SC_ADVANCED
 extver   = SlXvFeatureTest()
 patxname = NULL
+xref     = <this parameter must be set>