changed Sys_Sleep functions to take microseconds instead of milliseconds (this may...

changed Sys_Sleep functions to take microseconds instead of milliseconds (this may mildly improve sleep accuracy on Linux dedicated servers)
modified the host main loop to check for server packets each iteration of the loop rather than at the server framerate (this doesn't accomplish much yet but has potential)
added sv_checkforpacketsduringsleep cvar to enable use of select() instead of Sys_Sleep()

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7020 d7cf8633-e32d-0410-b094-e92efae38249