box: do not log option value if it not changed

After commit 9b2b3e58 ("box: apply dynamic cfg even if option
value is unchanged"), running box.cfg{} with any parameters logs
the values of those options, even if the option value has not changed.
This is quite awkward for config as it gives a lot of options to
box.cfg{}, although many of them may have the old value. This patch
causes box.cfg{} to log only those options whose values have changed.

Closes #9195

NO_DOC=No need.
NO_CHANGELOG=No need since the mentioned commit was not released yet.