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/mm0/include/task.h

@@ -48,6 +48,10 @@ struct tcb {
 	/* Related task ids */
 	unsigned int pagerid;	/* Task's pager */
 
+	/* Task's main address space region, usually USER_AREA_START/END */
+	unsigned long start;
+	unsigned long end;
+
 	/* Page aligned program segment marks, ends exclusive as usual */
 	unsigned long text_start;
 	unsigned long text_end;
@@ -69,8 +73,7 @@ struct tcb {
 	unsigned long map_end;
 
 	/* UTCB information */
-	unsigned long utcb_address;
-	int utcb_mapped;
+	void *utcb;
 
 	/* Virtual memory areas */
 	struct list_head vm_area_list;