support DEB_BUILD_PROFILES and -P for build profiles

support DEB_BUILD_PROFILES and -P for build profiles

Inspired by the rest of the patch in 661537, but abstract the
parsing of various ways of setting the build profiles more so it can
potentially be reused and all apt parts have the same behaviour.

Especially config options, cmdline options and environment will not be
combined as proposed as this isn't APTs usual behaviour and dpkg doesn't
do it either, so one overrides the other as it normally does.