Revert "sys_halt"

This reverts commit ff1d32885e98b7b0909576330929e4f2192b4a8b.
Merge branch 'a_pj2' of github.com:ccelest/pintos into a_pj2
Merge branch 'a_pj2' of github.com:ccelest/pintos into a_pj2
Merge branch 'a_pj2' of github.com:ccelest/pintos into a_pj2
Merge branch 'a_pj2' of github.com:ccelest/pintos into a_pj2

Conflicts:
	src/userprog/syscall.c
Merge branch 'a_pj2' of github.com:ccelest/pintos into a_pj2

Conflicts:
	src/userprog/syscall.c