lslogins: Fix discrepancies of SYS_UID_MIN

lslogins: Fix discrepancies of SYS_UID_MIN

util-linux does not contain useradd. Its most popular implementation
comes from shadow. SYS_UID_MIN is one of common parameters. Its
hardcoded fallback value is equal to 101 in shadow useradd (see
shadow-4.6/libmisc/find_new_uid.c: get_ranges()), but 201 in
login-utils/lslogins.c.

Let lslogins use the same fallback as useradd from shadow.

Hopefully most distros define its custom value of SYS_UID_MIN in
/etc/login.defs, so this problem is not visible.

login-utils/lslogins.1 does not mention its default at all. Add a
reference and improve text of lslogins(1) to prevent off-by-one
interpretation.

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>
Signed-off-by: Karel Zak <kzak@redhat.com>