Fixes #3896

strings.HasPrefix is more correct; if a tag part value ended up
containing the expected prefix of another part, it could cause incorrect
parsing. I don't think that these values would be semantically legal
today, but it's probably better to be defensive.

* Differentiate between user/internal error in AppRole login.

This allows us to properly pass through internal errors back up into
core.

* Separate out error cases

* Don't run rollback and upgrade functionality if we are a replication
secondary, but do if the mount is local.

This is useful for deciding when to run upgrade logic, e.g. if on a
performance secondary but local it's fine to run.

* plugins/gRPC: fix issues with reserved keywords in response data

* Add the path raw file for mock plugin

* Fix panic when special paths is nil

* Add tests for Listing and raw requests from plugins

* Add json.Number case when decoding the status

* Bump the version required for gRPC defaults

* Fix test for gRPC version check

* remove merge conflict remnant

* s/auth/authentication

* Handle period's zero value on handleCreateCommon

* Add test for period zero value

* website: add note about the 0.9.2+ CLI changes to reduce confusion

* website: fix frontmatter for 0.9.3 guide, add to guides index

* website: add overview title to 0.9.3 guide for spacing

Fixes #3858