* NEWS, lib/getdef.c, man/login.defs.5.xml: New login.defs

variable: MAX_MEMBERS_PER_GROUP. Used for the split groups support.
* lib/commonio.c, lib/commonio.h: Add an open_hook and close_hook
  operation. They are called after the database is actually opened
  and parse, or before it is closed.
* lib/groupio.c: Add an open_hook to merge split groups, and an
  close group to split groups if MAX_MEMBERS_PER_GROUP is set.
  This fixes gpasswd and chgpasswd when split groups are used.
* lib/sgroupio.c, lib/shadowio.c, lib/pwio.c: No open or close
  hooks for these databases. (unsure about what should be the gshadow
  behavior for split groups)

lib/commonio.h

@@ -52,6 +52,15 @@ struct commonio_ops {
 	 */
 	char *(*fgets) (char *, int, FILE *);
 	int (*fputs) (const char *, FILE *);
+
+	/*
+	 * open_hook and close_hook.
+	 * If non NULL, these functions will be called after the database
+	 * is open or before it is closed.
+	 * They return 0 on failure and 1 on success.
+	 */
+	int (*open_hook) (void);
+	int (*close_hook) (void);
 };
 
 /*