Remove dead code.

	* libmisc/find_new_sub_gids.c: Remove dead code.
	find_new_sub_gids() is always called with *range_count set to 0.
	It's more difficult to keep the subordinate GIDs and UIDs
	synchronized, than for the user's UID/GId because the count of
	subordinate IDs may differ.
	* libmisc/find_new_sub_uids.c: Likewise.
	* lib/subordinateio.h, lib/subordinateio.c: Remove APIs that are
	no more needed: is_sub_uid_range_free(), is_sub_gid_range_free(),
	is_range_free().
Fix boundary conditions.

	* lib/subordinateio.c (find_free_range): max is allowed for new
	ranges.
Fix parse of ranges.

	* src/usermod.c: Fix parse of ranges. The hyphen might be followed
	by a negative integer.
Improve diagnostic.

	* src/usermod.c: Check early if /etc/subuid (/etc/subgid) exists
	when option -v/-V (-w/-W) are provided.
Improve documentation.

	* man/login.defs.d/SUB_GID_COUNT.xml: Document newusers behavior
	when the user already have subordinate group IDs.
	* man/login.defs.d/SUB_UID_COUNT.xml: Likewise.
	* man/login.defs.d/SUB_GID_COUNT.xml: Fix typo (MAX<->MIN).
	* man/login.defs.d/SUB_UID_COUNT.xml: Likewise.