Adds a new include, doc/cmd_help.h.  This is generated by
pandoc from doc/interactive-help.md.  It's ugly, but that's
why it's programatically generated.

CMake will either generate a new cmd_help.{h,man} or copy them
in from cmd_help.{h,man}.in if pandoc isn't available.

Update cmd.c to use all of these new #defines.  (It looks
cleaner now...!)

Remove the old get-interactive-commands.bash, which is no longer
a part of the build process.  pandoc also generates a man page
snippet for insertion by help2man.

lintian's man page check is picky enough that it was
grousing about:

1) "'`f_min`' and '`f_max`' are ..." occasionally appearing at the
   start of a line, thus being interpreted as groff macros due to
   the single quote.

2) The calibrate lms rxvga2 syntax line was not always wrapping
   properly.