San: Rename 'guard' to 'san'

This prepares the foundation for more sanitizer-related work in the
future.

test/unit/double_free.c

@@ -1,5 +1,5 @@
 #include "test/jemalloc_test.h"
-#include "test/guard.h"
+#include "test/san.h"
 
 #include "jemalloc/internal/safety_check.h"
 

test/unit/retained.c

@@ -1,6 +1,6 @@
 #include "test/jemalloc_test.h"
 
-#include "jemalloc/internal/guard.h"
+#include "jemalloc/internal/san.h"
 #include "jemalloc/internal/spin.h"
 
 static unsigned		arena_ind;

test/unit/san.c

@@ -1,8 +1,8 @@
 #include "test/jemalloc_test.h"
 #include "test/arena_decay.h"
-#include "test/guard.h"
+#include "test/san.h"
 
-#include "jemalloc/internal/guard.h"
+#include "jemalloc/internal/san.h"
 
 static void
 verify_extent_guarded(tsdn_t *tsdn, void *ptr) {