Doc/Makefile: give mansource/-version/-manual attributes
Rather than hardcoding "Git Manual" and "Git" as the manual and source
in asciidoc.conf, provide them as attributes `manmanual` and
`mansource`. Rename the `git_version` attribute to `manversion`.
These new attribute names are not arbitrary, see, e.g., [1].
For AsciiDoc (8.6.10) and Asciidoctor <1.5.7, this is a no-op. Starting
with Asciidoctor 1.5.7, `manmanual` and `mansource` actually end up in
the xml-files and eventually in the rendered manpages. In particular,
the manpage headers now render just as with AsciiDoc.
No versions of Asciidoctor pick up the `manversion` [2], and older
versions don't pick up any of these attributes. -- We'll fix that with a
bit of a hack in the next commit.