[kernel] Remove process & thread self-handles

For the coming switch to cap/handle ref-counting being the main lifetime
determiner of objects, get rid of self handles for threads and processes
to avoid circular references. Instead, passing 0 to syscalls expecting a
thread or process handle signifies "this process/thread".

src/libraries/libc/arch/amd64/crt/crt0.s

@@ -5,15 +5,21 @@ extern __init_libc
 
 global _start:function (_start.end - _start)
 _start:
-	mov rbp, rsp
-	mov rdi, rsp
-	call __init_libj6
-	call __init_libc
+    push 0              ; Add null frame
+    push 0
+    mov rbp, rsp
 
-	pop rdi
-	mov rsi, rsp
-	call main
+    call __init_libj6
+    mov rbx, rax
 
-	mov rdi, rax
-	call exit
+    call __init_libc
+
+    pop rdi
+    mov rsi, rsp
+    mov rdx, 0    ; TODO: actually parse stack for argc, argv, envp
+    mov rcx, rbx
+    call main
+
+    mov rdi, rax
+    call exit
 .end:

src/libraries/libc/arch/amd64/crt/init.cpp

@@ -11,11 +11,14 @@ namespace {
 void
 run_ctor_list(cb *array, cb *end)
 {
+    if (!array || !end)
+        return;
+
     size_t i = 0;
     while (true) {
         const cb &ctor = array[i++];
         if (&ctor == end) return;
-        ctor();
+        if (ctor) ctor();
     }
 }