Upstream changes:
- Fixed: Don't fail when building the manual with AsciiDoc and xmllint.
- Fixed: Some long options (--rule, --rules-file and --rules-data) were
ignored.
- Fixed: Manpage formatting is now more consistent.
- Added: A new --version command line argument for showing version
information.
With the new upstream release recommending to use AsciiDoc to generate
the manpage, I also switched to AsciiDoc instead of Asciidoctor. The
resulting manpage looks better in some ways (for example when definition
lists are used).
I also added an installCheckPhase to ensure that the manpage exists in
the resulting store path.
Signed-off-by: aszlig <aszlig@nix.build>