configure: Choose highest-numbered version of tcl/tk

Otherwise, on my system (debian testing) which happens to have both 8.5
and 8.6 installed, as luck would have it tcl8.6 and tk8.5 were chosen
and configure complained about it.
rtapi: use proper type for rtapi_print_msg level
Remove unused define
milltask: don't link with ULAPISRCS

.. that's just the same stuff that's in liblinuxcnchal.so
configure: make missing img::png a configure-time failure
packaging: must build-depend on what configure checks for
Docs: 4 dashes before and after denote a code block

Signed-off-by: John Thornton <jthornton@gnipsel.com>
Docs: fix column alignment

Signed-off-by: John Thornton <jthornton@gnipsel.com>
Component: fix incorrect calculation of velocity tolerance percent

Signed-off-by: John Thornton <jthornton@gnipsel.com>
Merge remote-tracking branch 'origin/2.6'