mk-sbuild needs better http proxy support

right now mk-sbuild has minimal support for using an apt proxy.
Looking at history, in theory this was fixed under bug 416438, but I don't really understand how.

during a 'mk-sbuild' run, such as:
  ./mk-sbuild --debootstrap-mirror http://us.archive.ubuntu.com/ubuntu --arch amd64 precise

There are at least 3 places where a proxy would be necessary:
 a.) initial invocation of debootstrap
   currently:
    sudo "$DEBOOTSTRAP_COMMAND" --arch="$CHROOT_ARCH" \
       $variant_opt $debootstrap_opts "$RELEASE" \
       "$MNT" "${DEBOOTSTRAP_MIRROR:-http://archive.ubuntu.com/ubuntu}"
 b.) the 'finish.sh' script that is invoked inside the chroot after initializing
 c.) subsequent uses of that schroot

The only place in which 'http_proxy' environment variable is respected as far as I can tell is in 'b'.
 a.) http_proxy environment does not make it through 'sudo' by default.
 c.) creating a new schroot session like:
     http_proxy=myfooproxy schroot --chroot precise-amd64 --begin-session --session-name foobar
     http_proxy=myfooproxy schroot --run-session --chroot foobar

     the http_proxy variable does not make it through in either case.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: ubuntu-dev-tools 0.132ubuntu0.1
ProcVersionSignature: Ubuntu 3.0.0-12.20-generic 3.0.4
Uname: Linux 3.0.0-12-generic x86_64
ApportVersion: 1.23-0ubuntu3
Architecture: amd64
Date: Tue Oct 25 16:00:44 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-dev-tools
UpgradeStatus: No upgrade log present (probably fresh install)