Minor tweaks to toplevel docs as well as some doc/*.txt,
doc/variables.txt renamed to doc/params.txt, and a brand new
doc/pkglists.txt is added (thanks manowar@ for his considerations).

Somewhat more uniform style and a few additional bits.

distcfg.mk is nice for tracing the variable values' build-up,
but it's more useful to have a look at the final result sometimes.
So here it is, logged for each build and callable by hand.