Relax key and username matching for sshd

See merge request gitlab-org/gitlab-shell!540

Due to the way sshd works, gitlab-shell could be called with a single
string in the form:

```
/path/to/gitlab-shell -c key-id
```

However, due to the tightening of the regular expressions in fcff692b
this string no longer matches, so logins would fail with:

```
Failed to get username: who='' is invalid
```

This can be reproduced by changing the user's shell to point to
gitlab-shell. For example:

```
usermod git -s /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell
```

While setting gitlab-shell as the user's shell isn't officially
supported, gitlab-shell still should be able to cope with the key being
specified as the last argument. We now split the argument list and use
the last value.

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/530

Add logging to handler/exec.go and config/config.go

See merge request gitlab-org/gitlab-shell!539

Improve logging for non-git commands

See merge request gitlab-org/gitlab-shell!538

Reject non-proxied connections when proxy protocol is enabled

Closes #532

See merge request gitlab-org/gitlab-shell!536

Several of our commands only touch the internal API, and go nowhere
near Gitaly. Improve logging for each of these in a single MR. In
general, we want to be able to tell what happened in the execution of
each command, and to track failures down to a specific line of code.

Changelog: added

Update to Go v1.16.9

See merge request gitlab-org/gitlab-shell!537

This is a security release: https://golang.org/doc/devel/release#go1.16

refactor: remove call to BuildNameToCertificate (deprecated)

See merge request gitlab-org/gitlab-shell!515

This will help to prevent misconfigurations.

Changelog: fixed

Log command invocation

See merge request gitlab-org/gitlab-shell!535

Use reflection to log the command we are about to execute, both in
gitlab-shell and gitlab-sshd. Include the environment, which has all
the context we need to understand what the command is expected to do.

Changelog: added

Fix logging channel type

See merge request gitlab-org/gitlab-shell!534

Currently we get this in some log messages:

logrus_error="can not add field \"channel_type\""

This is because we're trying to add a function, rather than the result
of the function call (a string) to the `log.Fields`. Whoops!

Changelog: fixed

Resolve an error-swallowing issue

Closes #499

See merge request gitlab-org/gitlab-shell!533

Add more logging to gitlab-sshd

See merge request gitlab-org/gitlab-shell!531

Respect log-level configuration again

Closes #502

See merge request gitlab-org/gitlab-shell!530

This was lost in the move from Ruby to Go. Restore it now.

Changelog: fixed

Only validate SSL cert file exists if a value is supplied

See merge request gitlab-org/gitlab-shell!527

This fixes a regression in
https://gitlab.com/gitlab-org/gitlab-shell/-/merge_requests/508. If an
HTTPS internal API URL were used, gitlab-shell would not work at all. We
now handle blank `caFile` properly.

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/529

makefile: properly escape '$' in VERSION_STRING

See merge request gitlab-org/gitlab-shell!525

refactor: unify instantiation of command.Shell

Closes #517

See merge request gitlab-org/gitlab-shell!520

Improve err message given when Gitaly unavailable

Closes gitlab#340819

See merge request gitlab-org/gitlab-shell!526

If git is not available or gitlab-shell is not built in a repository, it falls back the VERSION file. That command is not properly escaped and results in the message:

> awk: cmd. line:1: Unexpected token

When you remove the `2>/dev/null`. Escape the '$' characters to solve this.

Add context fields to logging

See merge request gitlab-org/gitlab-shell!524

It adds correlation ids wherever possible

Extract server config related code out of sshd.go

Closes #523

See merge request gitlab-org/gitlab-shell!523

Unit test exit-codes for sshd/session.go

Closes #522

See merge request gitlab-org/gitlab-shell!522