lev_comp: Fix system RNG use in randline

The use of the system RNG (rand) for randline instead of the in-game RNG
caused games that reached levels that used it like the Town/Gehennom
fill levels to become unreplayable, e.g. the same level rewinded and
visited would appear different each time, with unrecoverable state
deviation thereafter.

It took me four days to track down why the hell this was happening.

This is meant to go with commit 319ca2d6d8 (CS652: one of the lev_comp
uber-commits porting 1.5k lines and 8-odd features worth of Sporkhack
changes into UnNetHack; I hate uber-commits).