DocBook manpage improvements

The DocBook code for generating manpages contained a few bugs. Probably for hysterical raisins, DocBook requires manpages (refentry documents) to use `refsection` instead of `section` (and `refsect1` instead of `sect1`, etc.). Also, the block element holding the synopsis section should be `refsynopsisdiv`.

Finally, man pages require some additional metadata in the `refmeta` and `refnamediv` elements immediately after the main document's `info` block. Without these, the DocBook stylesheets won't be able to determine the manpage name and section, and will try to output to a file called `.1`.

This is the minimal amount of changes required on the Asciidoctor side to build Git using Asciidoctor instead of AsciiDoc. I will be sending a patch series to the Git list to fix up the Git side of things.