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/include/exregs.h

@@ -6,6 +6,7 @@
 void exregs_set_stack(struct exregs_data *s, unsigned long sp);
 void exregs_set_mr_return(struct exregs_data *s, unsigned long retreg);
 void exregs_set_pc(struct exregs_data *s, unsigned long pc);
+void exregs_set_pager(struct exregs_data *s, l4id_t pagerid);
 
 /*
 exregs_set_stack(unsigned long sp)

tasks/mm0/include/memory.h

@@ -21,7 +21,4 @@ void init_mm_descriptors(struct page_bitmap *page_map,
 			 struct bootdesc *bootdesc, struct membank *membank);
 void init_physmem(struct initdata *initdata, struct membank *membank);
 
-int do_mmap(struct vm_file *mapfile, unsigned long f_offset, struct tcb *t,
-	    unsigned long map_address, unsigned int flags, unsigned int pages);
-
 #endif /* __MEMORY_H__ */

tasks/mm0/include/mmap.h

@@ -26,8 +26,8 @@ struct vm_area *vma_new(unsigned long pfn_start, unsigned long npages,
 
 int do_munmap(void *vaddr, unsigned long size, struct tcb *task);
 
-int do_mmap(struct vm_file *mapfile, unsigned long f_offset, struct tcb *t,
-	    unsigned long map_address, unsigned int flags, unsigned int pages);
+void *do_mmap(struct vm_file *mapfile, unsigned long f_offset, struct tcb *t,
+	      unsigned long map_address, unsigned int flags, unsigned int pages);
 
 int mmap_address_validate(struct tcb *t, unsigned long map_address,
 			  unsigned int vm_flags);