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)
