Fix some rather interesting bugs that managed to not turn up in various

testing and real-life applications:
1) If you returned from the thread function, you got a segv instead of
  calling _exit() with your return code.
2) clean up some bogus stack management.  There was also an underflow
  on function return.
3) when making syscalls, the kernel is expecting to have to leave space
  for the function's return address.  We need to duplicate this.  It was
  an accident that the rfork syscall actually worked here. :-/

lib/libc/i386/gen/rfork_thread.S

@@ -63,9 +63,8 @@ ENTRY(rfork_thread)
 	/*
 	 * Prepare and execute the thread creation syscall
 	 */
-	pushl	12(%ebp)
 	pushl	8(%ebp)
-	pushl	%esi
+	pushl	$0
 	leal	SYS_rfork, %eax
 	KERNCALL
 	jb 	2f
@@ -91,16 +90,17 @@ ENTRY(rfork_thread)
 	movl	%esi,%esp
 	popl	%eax 
 	call	*%eax
-	addl	$12, %esp
+	addl	$4, %esp
 
 	/*
 	 * Exit system call
 	 */
 	pushl	%eax
+	pushl	$0
 #ifdef SYS_exit
-	pushl	$SYS_exit
+	leal	SYS_exit, %eax
 #else
-	pushl	$SYS_sys_exit
+	leal	SYS_sys_exit, %eax
 #endif
 	KERNCALL