targets/core: Set proxy variables based on previous proxy setting

On update, obtain `*_proxy` variables from the previously generated environment, and export them after sanitizing the value (to make sure we do not eval random harmful stuff).

Fixes #883.

Tested in `2014-07-04_04-33-53_drinkcat_chroagh_fix-proxy-env/peppy-beta_2f863267` (that is, `squawks` completed, which should be good enough).
I think it's being overcautious.  Someone with root access inside the chroot (which is necessary for modifying /etc/environ) could just as easily replace the sed executable...
As long as the sourcing of a root-only file happens within the chroot (and not in a script being run outside the chroot), I think it's OK, and is preferable over having a sed line that looks like that...

As far as breakage goes, we can either admit that misconfiguration of apt already breaks the update process and /etc/environ should be no different, or we can first attempt sourcing it in a (subshell) and spit out an error message if that fails before doing it for real.