Fix respawn/spawn timers racing ahead under multi-threading

POSIX clock() sums CPU time across every thread in the process, unlike
Windows' clock() which tracks wall-clock milliseconds. With several
worker threads running, respawn/spawn/AI timers based on clock() raced
far ahead of real elapsed time. Switch spawn-timer bookkeeping to
time(NULL)/time_t (seconds) throughout the world server, and add a
portable clock() shim in sockets.h (backed by CLOCK_MONOTONIC,
millisecond resolution) for remaining call sites so CLOCKS_PER_SEC==1000
still holds like it did on Windows.

Also: fix MapDelay/VisualDelay/WorldDelay being passed to usleep() (us)
without converting from their configured milliseconds; fix the shutdown
timer likewise; stop leaking the temporary actor used to run an NPC's
AIP script; fix CleanDrops() skipping the element that shifts into
position after an erase.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Src/World Server/worldmap.h

@@ -93,7 +93,7 @@ class CMap
     UINT TDMobList[500];        //List of TD monsters waiting to be spawned
     UINT TDNextSpawn;           //index pointer for next spawn in TDMobList
     UINT TDNextMon;             //index pointer for next addition to TDMobList
-    clock_t lastTDSpawnTime;    //time of the last spawn
+    time_t lastTDSpawnTime;     //time of the last spawn
     UINT TDMobDelay;            // delay between spawns (ms)
     UINT crystalhealth;         // current health of the base crystal
     UINT portalhealth;          // current health of the spawn portal