Mostly multiline compactions, but also use/install2/net
got factored out from distro/server-ovz for future reuse.

Preferences might be somewhat interesting too: while the official
ones shouldn't influence the build result at all, there's no whitelist
so all kinds of weirdness can be stuffed into local config in principle.
That should be diagnosable at least.

This was done while debugging GLOBAL_CLEANUP_PACKAGES
getting doubled... as it got no hard initial value but
rather was always added to, it appeared that at least
stage2/Makefile would obtain a once-initialized value
from upstream and double it while including distcfg.mk
itself.

It's way less hassle to just proxy the value once.

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.