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/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__ */