implemented individual file downloads on darkplaces servers

implemented individual file downloads on darkplaces servers
deferred loading of files until signon 2 (necessary for download system)
made loading of files operate in two passes, first for existing files and then again for files that need to be downloaded
added cl_joinbeforedownloadsfinish cvar to cause the game to begin after the map is loaded, before other downloads finish (not entirely sure this is a good idea, but it's cool)
added sv_allowdownloads cvar to allow enabling/disabling of downloads, and sv_allowdownloads_inarchive and sv_allowdownloads_archive cvars to allow server administrators to enable downloading files from archives or even whole archives if so desired (however the client does not currently request archives)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6715 d7cf8633-e32d-0410-b094-e92efae38249