* daemon: add offline field to remodeling JSON payload
* client, cmd/snap: add offline boolean option for remodeling to client and cli interface
* daemon: make snap files optional in the "multipart/form-data" remodel api
* tests/nested/manual/remodel-offline: add variants to test different combinations of installed/local snaps/assertions
* client: update comment on RemodelWithLocalSnaps to mention not contacting the store
* c/snap: remove unneeded var declaration
* tests/nested/manual/remodel-offline: clarify section that acks/adds the validation set assertion
* tests/nested/manual/remodel-offline: fix shellcheck warning on test's -a flag
* client: verify that normal (online) remodel does not set offline field in JSON payload