New config API with TOML

[stevapple]

This PR adds a new config API base on TOML, aiming for replacing the existing JSON one. The new config API has adopted the following changes:

• Thanks to TOML, we're able to group the config items in a natural way;
• logger has gained a switch and now requires an endpoint URL, preparing for the upcoming OpenTelemetry support;
• host-keys can be hard-coded alternatively in base64 or string format;
• DNS names are now supported to determine if PROXY protocol is allowed;
• Some of the config keys are renamed.

Tests are updated to reflect these changes. They now uses static fixtures, eliminating the requirement of a /tmp/sshmux directory and ssh-keygen tool. The JSON APIs are deprecated but not removed, so tests are now performed against both formats.

This PR also adds SO_REUSEPORT support to sshmux, unblocking #1.

[stevapple]

@iBug Request for an early review before the documentation is finished.

We'd like to take advantage of the expressivity of TOML to unblock more and more flexible configurations.

[iBug]

Looks great so far. Nothing to complain about. Keep up with your good work!

[stevapple]

I have to apologize that this PR goes far beyond simply adding an alternative config API, but it's not an easy task to split all the updates off now.

I would try some time tomorrow if I have time. If it didn't work, we'll have to squash all those stuffs in a single commit.

[stevapple]

I would try some time tomorrow if I have time.

I believe the commit history is now clean enough for a fast-forward merge.