Add definition of nitems for those without it and use it throughout.

include/sudo_compat.h

@@ -237,6 +237,13 @@ typedef struct sigaction sigaction_t;
 # define SA_RESTART	0
 #endif
 
+/*
+ * The nitems macro may be defined in sys/param.h
+ */
+#ifndef nitems
+# define nitems(_a)	(sizeof((_a)) / sizeof((_a)[0]))
+#endif
+
 /*
  * If dirfd() does not exists, hopefully dd_fd does.
  */