Rewrite d/rules from scratch and document our changes

INTRO

d/rules contains a lot of stuff that is outdated and it's time to rewrite it.

GOAL1

The main goal is to make d/rules easy to read even for people who don't work with nginx every day. And unify the use of parameters that are stored in nginx-dev (--override-* and --with-cc-opt,--with-ld-opt, ...)

GOAL2

A second goal is to document our settings, such as --override-*, something that is described in this issue.