feat: add process termination codes and SYS_ABORT; whole-process abort() from any thread in libc and montauk-rt

kernel/src/Sched/Scheduler.cpp

@@ -1885,7 +1885,7 @@ namespace Sched {
                 continue;
             }
 
-            primary.exitCode = 256 + 9; /* killed (SIGKILL) */
+            primary.exitCode = TerminatedKilled;
             primary.killPending = true;
             if (primary.state == ProcessState::Blocked) {
                 primary.state = ProcessState::Ready;

kernel/src/Sched/Scheduler.hpp

@@ -6,6 +6,7 @@
 
 #pragma once
 #include <cstdint>
+#include <Common/TerminationCodes.hpp>
 #include <atomic>
 #include <Api/Syscall.hpp>
 
@@ -303,9 +304,9 @@ namespace Sched {
 
     // Kill a process by PID. If the process is running on another CPU,
     // sets a kill-pending flag checked on the next timer tick. exitCode is
-    // what a waiter sees (256+sig; SIGKILL by default).
+    // what a waiter sees (a TerminationCodes value).
     // Returns 0 on success, -1 on failure.
-    int KillProcess(int pid, int exitCode = 256 + 9);
+    int KillProcess(int pid, int exitCode = TerminatedKilled);
 
     // Start a new process session for the caller, or terminate every process
     // belonging to a session. KillSession returns the number of live members