It's far more complicated than people would want to believe. In most cases it is better done as a client-side modification (see: SauerbratenEnhanced). Better still, Blood Frontier has all this and more, as it is more targeted toward an arena-style shooter, but it is a standalone game separate from Sauerbraten.
1) Considering that rays collide with a full-radius bounding box, headshot recognition is tricky without defining separate bounding boxes for various parts (eg. the headshot bounding box should be quite small). It's not really a matter of saying "if z-axis is between these two points it is a headshot", because it could have gone off to either side and still made contact with the bounding box (see: /showboundingbox 1).
2) Again, better as a client side modification inside fpsgame/fps.cpp inside the killed() function; requires tracking of kill times and numbers.
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