vacuumlo - use a cursor

vacuumlo is rather simpleminded about dealing with the list of LOs to be 
removed - it just fetches them as a straight resultset. For one of my 
our this resulted in an out of memory condition. The attached patch 
tries to remedy that by using a cursor instead. If this is wanted I will 
add it to the next commitfest. The actualy changes are very small - most 
of the patch is indentation changes due to the introduction of an extra 
loop.

cheers

andrew