[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/kernel/syscalls/thread.cpp

@@ -17,9 +17,14 @@ thread_create(j6_handle_t *self, process *proc, uintptr_t stack_top, uintptr_t e
     thread &parent_th = thread::current();
     process &parent_pr = parent_th.parent();
 
+    if (!proc)
+        proc = &parent_pr;
+
     thread *child = proc->create_thread(stack_top);
     child->add_thunk_user(entrypoint, arg0, arg1);
-    *self = child->self_handle();
+
+    *self = g_cap_table.create(child, thread::creation_caps);
+
     child->set_state(thread::state::ready);
 
     log::verbose(logs::task, "Thread <%02lx:%02lx> spawned new thread <%02lx:%02lx>",

src/kernel/syscalls/vm_area.cpp

@@ -31,14 +31,16 @@ vma_create_map(j6_handle_t *self, size_t size, uintptr_t base, uint32_t flags)
 j6_status_t
 vma_map(vm_area *self, process *proc, uintptr_t base)
 {
-    proc->space().add(base, self);
+    vm_space &space = proc ? proc->space() : process::current().space();
+    space.add(base, self);
     return j6_status_ok;
 }
 
 j6_status_t
 vma_unmap(vm_area *self, process *proc)
 {
-    proc->space().remove(self);
+    vm_space &space = proc ? proc->space() : process::current().space();
+    space.remove(self);
     return j6_status_ok;
 }