exchange_registers(), modified thread_control calls seem to work

- Fixed do_mmap() so that it returns mapped address, and various bugs.
- A child seems to fork with new setup, but with incorrect return value.
  Need to use and test exregs() for fork + clone.
- Shmat searches an unmapped area if input arg is invalid, do_mmap()
  should do this.

tasks/mm0/src/arch-arm/exregs.c

@@ -7,6 +7,12 @@
 #include <exregs.h>
 
 
+void exregs_set_pager(struct exregs_data *s, l4id_t pagerid)
+{
+	s->pagerid = pagerid;
+	s->flags |= EXREGS_SET_PAGER;
+}
+
 void exregs_set_stack(struct exregs_data *s, unsigned long sp)
 {
 	s->context.sp = sp;