Merge branch 'master' into deploy

* master:
  Makefile: Add `help' summary.
  Makefile: Float `V' along with the other configuration variables
  bin/chroot-maint: Set HTTPS proxy along with the HTTP proxy.
  Makefile, etc/aptsrc.conf: Update for new Debian releases.
  Makefile: Force creation of directories using an order-only pattern rule.
  Makefile: Introduce and use some improved pathname manipulation functions.
  Makefile: Don't create random directories in the working tree.
  Makefile: Make `all' and `schroot-config' be single-colon rules.