box: store box.cfg.force_recovery in C

box.cfg.force_recovery used to be needed only during box.cfg() in
a few places, but its usage is going to extend.

In future commits about cluster/replicaset/instance names it will
be needed to allow rename. It won't be entirely legal (hence can't
be done without any flags), but won't be fully illegal either.

The "valid" rename will be after upgrading, when an
old cluster updated to a new version and wants to start using the
names. Then it will have to set force_recovery, set the names,
sync the instances, drop force_recovery. One-time action to allow
old installations use the new feature - the names.

Part of #5029

NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=already covered