This fixes #153 by making all platform dependencies be features and including them as default features, so now you can suppress building all platform dependencies by suppressing default features.

This also fixes #148 by having the code check to see if there is platform support and, if not, using the mock keystore as the default.

While this change is not fully backward compatible for people who were suppressing default features on macOS, iOS, Win, or FreeBSD, doing so was undefined behavior on those platforms and so changing it does not break the semantics of the API.

Rework default feature set.

This came about as a build fix for OpenBSD's new spotifyd package:
```
error[E0433]: failed to resolve: use of undeclared crate or module `default`
  --> .../modcargo-crates/keyring-2.0.1/src/lib.rs:176:54
```

Local spotifyd patches to enable its keyring feature practically differ
only in README.md and Cargo.toml churn;  this makes keyring-rs build fine
by adapting existing FreeBSD bits.

Support OpenBSD

Also update dependencies.

Thanks to @klemensn's #156 this release should fix #157.

Update release to support OpenBSD.