Patch to allow different replacement modes/behaviors

Attached is a patch that makes the replace mode user-configurable. The default mode is the way Upload Replace works now. The other mode is "replace" meaning that old files aren't versioned but overwritten.

I wrote this patch to meet a need a client of ours had. They needed to have several nodes referencing the same file but wanted the last uploaded version of a file to overwrite any previous versions and keeping URLs to files consistent. This patch maintains existing file table entries but replaces the file and updates the file size so that several file table entries refer to the same file.