CLEANUP: Replace weak RNGs with arc4random, make NOSPOOF unconditional

CLEANUP: Replace weak RNGs with arc4random, make NOSPOOF unconditional

Replace srand/rand (NOSPOOF seeds) and srand48/lrand48 (DNS query IDs)
with arc4random(), which is cryptographically strong and needs no seeding.

Simplify NOSPOOF anti-spoof challenge: replace the MD5-based construction
(which depended on predictable seeds) with a direct arc4random() call.
Remove NOSPOOF_SEED01/NOSPOOF_SEED02 globals and MD5 dependency.

Make NOSPOOF unconditional — every modern IRC client handles PING-on-connect.
Remove all #ifdef NOSPOOF conditionals across the codebase.

Fix operator precedence bug in res.c where & 0xffff only applied to the
lrand48() result instead of the whole expression.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>