Make asciidoc optional

Asciidoc can be a fairly large download as a dependency. In order to let this be an optional dependency, this patchset adds an alternative help which uses less with the asciidoc markdown source files.

This patchset also adds a man_pages meson config setting. If set to the default of auto, it will try to use asciidoc if found, but fallback to less if asciidoc is not found.

If man_pages is enabled, the asciidoc is a required dependency, and if disabled, then it will not be searched for.