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.

include/l4/api/exregs.h

@@ -11,10 +11,16 @@
 #include INC_GLUE(context.h)
 #include <l4/types.h>
 
+enum exregs_flags {
+	EXREGS_SET_PAGER = 1,
+};
+
 /* Structure passed by userspace pagers for exchanging registers */
 struct exregs_data {
 	exregs_context_t context;
 	u32 valid_vect;
+	u32 flags;
+	l4id_t pagerid;
 };