Add extension support

[nigoroll]

This is the groundwork to add vtc commands at runtime, as required for making it possibly to use an externally packaged vtest in vinyl-cache.

The first commit adds dynamic registration of top level commands: Rather than using a constant array of commands, we maintain a dynamic array to which registered commands are appended.

The second commit adds the minimal dlopen() scaffolding to load extensions and adds a demo-extension called foo which adds a command by the same name.

Ref: varnishcache/varnish-cache#4398

[bsdphk]

Due to a misunderstanding, I was not aware of this PR when I started implementing the same basic idea.

However, this PR did also suffer from a number of problems, most importantly doing dlopen() before the fork() which protects the vtest-core against the tests going off the rails.

The committed version also has an autodiscovery facility, so users, in the normal case, will not have to think about proper -E arguments.