docs/man: Fix/simplify generation of manpages

docs/man: Fix/simplify generation of manpages

The original intent of this patch was to rename xen-vbd-interface.markdown.7
to xen-vbd-interface.pandoc.7 to remove the final markdown file from the docs/
tree.

The DOC_MANx lists are broken.  They contain MANxSRC-y twice, the first half
with a partial %.pod substituation, and the second half with a partial
%.markdown substitution.  This is also the root cause behind the filtering
activity in the uninstall-man$(i)-pages rule.

Furthermore, the logic for generating the manpage targets is unnecesserily
repetative, owing to the layout of source files in the man/ directory.

Therefore, tackle the problem by renaming all of our manpage source files from
"$FORMAT.$SECTION" to "$SECTION.$FORMAT".  For the two xl.cfg.5 and xl.1 which
are preprocessed by autoconf to contain path information, this requires
updating configure.ac and .gitignore.  The markdown to pandoc conversion is
performed as well, as it is also a straight rename.

An ancillary benefit of this renaming is that text editors stand a chance of
being able to work out the correct mode to use.

As for the makefile:

1) Break the MAN_SECTIONS list out of the GENERATE_MANPAGE_RULES loop, as we
   are going to use it a second time.
2) Do away with the individaul MANxSRC-y variables.  Use a single list,
   derived from all *.pod and *.pandoc files, with their format suffixes
   removed.
3) Use a $(foreach ...) to generate the DOC_MANx lists, filling them with the
   correct content.
4) The DOC_HTML and DOC_TXT can now include all manpages with a single
   substitution, as they don't need to separate the manpages by
   section-numbered-directory.
5) Fix up the filenames in the manpage metarule to match the renaming.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>