Rewrite `login` and registry cleanups.

- Login:
    - Change `login` so that it reads the API host from the registry config so it knows the `{api}/me` URL to display.
    - The `--host` flag is deprecated/unused. It wasn't used for much before.
    - `--registry` supports interactive entry of the token (does not require it on the command line).
    - Displays a message after saving the token (fixes #5868).
    - Because `login` now requires an index, some of the tests had to be updated.
- Fix so that if `api` is missing from the config, it will display a nice error message instead of panicking with unwrap.

Update cargo

24 commits in 0d1f1bbeabd5b43a7f3ecfa16540af8e76d5efb4..34320d212dca8cd27d06ce93c16c6151f46fcf2e
2018-12-19 14:45:14  0000 to 2019-01-03 19:12:38  0000
- Display environment variables for rustc commands (rust-lang/cargo#6492)
- Fix a very minor race condition in `cargo fix`. (rust-lang/cargo#6515)
- Add a high-level overview of how `fix` works. (rust-lang/cargo#6516)
- Add dependency `registry` to `cargo metadata`. (rust-lang/cargo#6500)
- Fix fingerprint calculation for patched deps. (rust-lang/cargo#6493)
- serialize version directly (rust-lang/cargo#6512)
- use DYLD_FALLBACK_LIBRARY_PATH for dylib_path_envvar on macOS (rust-lang/cargo#6355)
- Fix error message when resolving dependencies (rust-lang/cargo#6510)
- use PathBuf in cargo metadata (rust-lang/cargo#6511)
- Fixed link to testsuite in CONTRIBUTING.md (rust-lang/cargo#6506)
- Update display of contents of Cargo.toml (rust-lang/cargo#6501)
- Update display of contents of Cargo.toml (rust-lang/cargo#6502)
- Fixup cargo install's help message (rust-lang/cargo#6495)
- testsuite: Require failing commands to check output. (rust-lang/cargo#6497)
- Delete unnecessary 'return' (rust-lang/cargo#6496)
- Fix new unused patch warning. (rust-lang/cargo#6494)
- Some minor documentation changes. (rust-lang/cargo#6481)
- Add `links` to `cargo metadata`. (rust-lang/cargo#6480)
- Salvaged semver work (rust-lang/cargo#6476)
- Warn on unused patches. (rust-lang/cargo#6470)
- don't write a an incorrect rustc version to the fingerprint file (rust-lang/cargo#6473)
- Rewrite `login` and registry cleanups. (rust-lang/cargo#6466)
- [issue#6461] Fix cargo commands list (rust-lang/cargo#6462)
- Restrict registry names to same style as package names. (rust-lang/cargo#6469)

Use registry.default for login/logout

This changes `cargo login` and `cargo logout` to use the registry configured at `registry.default` as the registry instead of crates.io. For `cargo login`, this was an unintentional regression from #6466. The documentation has always stated that it will use the default registry.

This makes the command more in line with other registry-involving commands. There are still some inconsistencies.

These commands use the default if not specified:

* `cargo init`
* `cargo new`
* `cargo owner`
* `cargo search`
* `cargo yank`
* `cargo publish` uses the default, but will also look at the `publish` field `Cargo.toml` and use that if the registry is not specified.

These commands would always use crates.io if `--registry` is not specified:

* `cargo login`
* `cargo logout`
* `cargo install`

I'm a bit uncertain how to proceed, since this is technically a breaking change particularly if someone has scripted it. I suspect that the number of users that use `registry.default` is very small, and those that script `cargo login` are even smaller, and thus the intersection is probably small or nonexistent. However, there is some risk here.