Merge branch 'system-calls' into 'user-programs'

# Conflicts:
#   src/userprog/process.c

Fix conflict in differing placeholder process_wait implementations

src/userprog/process.c

@@ -14,6 +14,7 @@
 #include "threads/flags.h"
 #include "threads/init.h"
 #include "threads/interrupt.h"
+#include "threads/synch.h"
 #include "threads/palloc.h"
 #include "threads/thread.h"
 #include "threads/vaddr.h"