box.ctl: add iproto_lockdown option to security

The new option is configured by `box.ctl.iproto_lockdown()`. It is
avalilable only in Enterprise Edition builds.

Needed for tarantool/tarantool-ee#585

NO_DOC=Will be added to Enterprise Edition
NO_CHANGELOG=see NO_DOC
NO_TEST=No logic is added to Community Edition, will be tested in
        Enterprise Edition