Add SLIF_CATEGORY for the server list, and a menu QC function to set it.

Add SLIF_CATEGORY for the server list, and a menu QC function to set it.

Callback:
float m_gethostcachecategory(float entry_id) {
	// Is supposed to use gethostcachestring() etc. and return a
	// category integer.
	// The server list is sorted by this category first if
	// SLSF_CATEGORIES is set in the sort flags.
}

From: Samual Lenks <samual@xonotic.org>

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12021 d7cf8633-e32d-0410-b094-e92efae38249
::stable-branch::merge=d93b11b3f40d1ca94ed20e70c7a2fcf95670df73