Multiple above-minor updates.

- fixed is_err(x), was evaluating x twice, resulting in calling a
function x twice.

- Divided task initialisation into multiple parts.
- MM0 now creates a tcb for itself and maintains memory regions of its own.
- MM0's tcb is used for mmapping other tasks' regions. MM0 mmaps and prefaults
  those regions, instead of the typical mmap() and fault approach used by
  non-pager tasks.
  For example there's an internal shmget_shmat() path to map in other tasks'
  shm utcbs. Those mappings are then prefaulted into mm0's address space using
  the default fault handling path.

- FS0 now reads task data into its utcb from mm0 via a syscall.
  FS0 shmat()s to utcbs of other tasks, e.g. mm0 and test0.

  FS0 then crashes, that is to be fixed and where this commit is left last.

tasks/fs0/include/task.h

@@ -16,6 +16,7 @@
 struct tcb {
 	l4id_t tid;
 	unsigned long utcb_address;
+	int utcb_mapped;		/* Set if we mapped their utcb */
 	struct list_head list;
 	int fd[TASK_OFILES_MAX];
 	struct id_pool *fdpool;
@@ -23,6 +24,11 @@ struct tcb {
 	struct vnode *rootdir;
 };
 
+static inline int task_is_utcb_mapped(struct tcb *t)
+{
+	return t->utcb_mapped;
+}
+
 struct tcb *find_task(int tid);
 int init_task_data(void);