Sort pk3s in PAKSUMS in the order they're listed and after other pk3s

Allows loading RTCW-MP pk3s in correct order, by listing pak0 before mp_pak#
Addons (pk3s not in PAKSUMS) take priority over pk3s listed in PAKSUMS, regardless of file names.
Only warn about missing or modified pk3s when first loading mod

It was warning at each map change.