add variable substitution capability for layer's tileindex

[assefa]

e-mail from list : Jeff de La Beaujardiere <jeff2005@SVS.GSFC.NASA.GOV>

 (1) no substitution occurs in TILEINDEX entries in the mapfile.
> Example: the following substitution works perfectly
>  DATA "/home/delabeau/wms/data/goes/%year%/%month%/%day%/%file%"
> but this one
>  TILEINDEX "/home/delabeau/wms/data/modis/tc/%year%/%month%/%day%/%file%"
> complains
>  msSHPOpenFile(): Unable to access file. 
> (/home/delabeau/wms/data/modis/tc/%year%/%month%/%day%/%file%)
> 
> I think the problem is in mapserv.c after line 281 (v4.6.1), where data,
> connection, filter and class are explicitly considered but tileitem is 
> ignored.

[assefa]

added the capability.