Fix removal of ranges.

	* lib/subordinateio.c: Fix removal of ranges. The database needs
	to be marked as changed or commonio_close will discard any change.
Fix handling of boundaries.

	* lib/subordinateio.c: Fix handling of boundaries.
	* libmisc/find_new_sub_uids.c: Likewise.
	* libmisc/find_new_sub_gids.c: Likewise.
Improve error reporting.

	* src/useradd.c: Change message in case of find_new_sub_uids /
	find_new_sub_gids failure. This complements the messages already
	provided by these APIs.
Improve documentation.

	* man/login.defs.d/SUB_GID_COUNT.xml: Document that the behavior
	of useradd and newusers depends on the existence of /etc/subgid.
	* man/login.defs.d/SUB_UID_COUNT.xml: Likewise for /etc/subuid.