Merge branch 'master' into deploy

Merge branch 'master' into deploy

* master:
  Makefile: Use buster's Qemu on stretch.
  Makefile, bin/chroot-maint: Allow source distribution selection for Qemu.
  bin/chroot-maint: Add missing format argument to diagnostic.
  Makefile, bin/chroot-maint: Keep `/usr/local/include' mostly empty.
  etc/sbuild.conf.in: Don't run complicated Debian testing by default.
  Makefile: Add a new thing for running arbitrary commands.
  Makefile: Change the tag for maintenance to be `MAINT'.
  Makefile: Fix `squote' to have the right number of `\'s.
  bin/chroot-maint: Run `ldconfig' after an update.
  bin/chroot-maint: Fix mistakenly indented file-contents literals.
  src/jobclient.c: Cope if the jobserver pipe is set nonblocking.
  bin/chroot-maint: Force `--no-merged-usr'.
  bin/chroot-maint: Update the path correctly when following a symlink.
  bin/chroot-maint: Don't forget to copy symlinks to directories.