docs: reorg how manpages are generated

This introduces a new Makefile variable named GENERATED_MANPAGES, holding
the list of manpages created by the build system.  Think of it like a
TARGETS for manpages.

Manpages in GENERATED_MANPAGES automatically get included in the manpage
index in the HTML docs, and in the LinuxCNC_Manual_Pages.pdf document.

GENERATED_MANPAGES currently just has $(COMP_MANPAGES) and
$(COMP_DRIVER_MANPAGES), so the behavior is exactly like before
this commit.  This new variable enables easily & cleanly adding more
manpages in the future: soon the missing wj200_vfd & linuxcnc manpages,
and eventually lui.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: switch to recursive expansion for manpage variables

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: include linuxcnc.1 in the manpage index and pdf

This manpage got dropped from the index a good long while ago, this adds
it back in.  (The manpage was always included in the .deb, just not in
the html and pdf index.)

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: include the wj200 manpage in the html & pdf docs

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: build index.incl cleaner & more completely

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: verify all manpages are in the html manpage index

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: select the man1 and man3 manpages correctly

The man1 .in files haven't been around since commit 4ab6fc67 in May of
2013, the the extra (non-.hal, non-.rtapi) man3 pages have been around
since commit ae880382 in July of 2011.

I guess we just never noticed that they didn't make it into the HTML
manpage index....

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: move the skeleton manpages out of the way

This means there's no need to filter out the skeletons from the manpage
list in the docs/src/Submakefile any more.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>