Makefile.am: Be more careful about what gets distributed.

I've just noticed that I'm shipping all sorts of things which I should
be generating at build time, such as configured versions of
`confsubst'.

Fix this idiocy by removing pkgdata_MUMBLE from EXTRA_DIST and adding
things to dist_pkgdata_MUMBLE instead.  This took a couple of iterations
to get right but I think we're there now.