Check that the new fields set with -u, -s, -l, -g, -f, -e, -d, and -c

Check that the new fields set with -u, -s, -l, -g, -f, -e, -d, and -c
differ from the old ones. If a requested new value is equal to the old
one, no changes will be performed for that field. If no fields are
changed, usermod will exist successfully with a warning. This avoids
logging changes to syslog when there are actually no changes.