Convert manpage DocBook

[mojavelinux]

DocBookRx is missing several visitors for properly converting a manpage written in DocBook. Implement the necessary visitors and ensure that the output has the required structure to convert from AsciiDoc to a manpage.

• input: https://git.linuxtv.org/media_tree.git/tree/Documentation/DocBook/media/v4l/func-ioctl.xml
• expected result: https://git.linuxtv.org/mchehab/asciidoc-poc.git/tree/func-ioctl.adoc

[mojavelinux]

Ideally, we should setup a test (or several) for this type of document to make sure we are hitting all the nodes. Then, it's just a matter of implementing them.

We may find that we need some changes in the built-in manpage converter to accommodate all the parts of a manpage. I'm open to making the necessary changes. This is a great opportunity for discovery.

[mojavelinux]

This seems related to #24.

[mojavelinux]

See #70 for a short term solution. A more complete solution will be needed in order to solve the issue.