Allow limited range for r_znear and cg_gunX, Y, Z

[zturtleman]

r_znear and cg_gunX, cg_gunY, cg_gunZ cvars are cheat protected. r_znear allows changing depth cut-off to show more of view weapon and cg_gunX, Y, Z cvars allow customizing weapon placement. Neither of these seem like a concern for cheating. Aside from setting r_znear high to see through wall and maybe moving view weapon far forward to use a target marker?

The ranges in Quake Live were reported by SolivaN to be:

r_znear | 0 to 4
cg_gunX | -10 to 10
cg_gunY | -10 to 10
cg_gunZ | -10 to 0

This change was requested by SolivaN at the ioquake forum.