introduce temporary aliases

An alias, that was at least once used with the new "tempalias" will not be
written to saved.cfg.

So, now there are four ways to create aliases:

1) alias (or "=")
  General alias, who's value gets saved at game exit in saved.cfg, and
  restored at the next restart - unless the alias was created at a time,
  where persistidents was zero (usually during execution of the game's own
  config files during startup).

2) const
  Creates an alias, that can not be changed. Makes it harder for users
  to destroy their config.

3) push
  Reassigns an alias with a new value. After "pop", the old value is
  restored. Allows the use of "local" aliases in scripts. Also avoids
  name conflicts.

4) tempalias
  Like "alias", but the value will not be written to saved.cfg.

The commit also changes aliases, written by the engine itself, to "temp":
gametimestart, __getmaprevisions

DOCREF tempalias

source/src/worldio.cpp

@@ -754,7 +754,7 @@ bool load_world(char *mname)        // still supports all map formats that have
     c2skeepalive();
 
     defformatstring(startmillis)("%d", millis_());
-    alias("gametimestart", startmillis);
+    alias("gametimestart", startmillis, true);
     startmap(mname);
     return true;
 }