Add partial support for SSH known hosts markers

The SSH `known_hosts` file parsing in Cargo did not previously support
markers. Markers are modifiers on the lines (`@cert-authority` and
`@revoked`) which denote special behavior for the details on that line.
Lines were skipped entirely.

This silent skipping of marker lines can be confusing to a user, who
sees that their command line Git/SSH client works for some repository,
but Cargo reports that no host key is found.

This change adds support for the `@revoked` marker. This marker denotes
that a key should be rejected outright. It is of limited use without
`@cert-authority` marker support. However, if it is present in a user's
`known_hosts` file, then Cargo definitely shouldn't accept that key and
probably shouldn't suggest that the user add it to their `known_hosts`
either.

The change also adds support for detecting `@cert-authority` markers in
`known_hosts` files. These lines cannot yet be used for host key
verification, but if one is found for a matching host, the user will be
informed that Cargo doesn't support `@cert-authority` markers in the
error message. Additionally, the user will be advised to use the
`net.git-fetch-with-cli` config option to use the command line git
client for fetching crates from Git.

Refs: rust-lang#11577