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/glue/arm/context.h

@@ -5,10 +5,10 @@
 
 /*
  * This describes the register context of each task. Simply set
- * them as regular structure fields, and they'll be copied onto
- * real registers upon a context switch to that task. Normally
- * exchange_registers() system call is designed for this, whose
- * input structure is defined further below.
+ * them and they'll be copied onto real registers upon a context
+ * switch to that task. exchange_registers() system call is
+ * designed for this, whose input structure is defined further
+ * below.
  */
 typedef struct arm_context {
 	u32 spsr;	/* 0x0 */