Performance fix in system_region_list() avoid an SQL query by using globals

Hello,

I did a really small patch that avoid one SQL in system_region_list() function. Because we have the current theme information available as a global variable, there is no use to do an SQL query to fetch it back (again) when asking the current theme region list.

I known that D7 needs the patch, then backport it to D6, but the system_region_list() has changed, and does not work the same anymore, so this is NOT about D7 here, this is a really small performance improvement for D6.