lab 4 fixes, from MIT JOS:
    * remove confusing comment
    * execution can continue past hlt, so loop infinitely
    * remove E_NO_SYS
Merge branch 'lab4' of http://news.cs.nyu.edu/~mwalfish/classes/14fa/jos into lab4
This is for the lab4 bug commit
Conflicts:
	kern/syscall.c
commit after pulling lab4 bugfix