Remove 'nix-upstream' submodule.

* nix/sync-with-upstream: Remove.
* bootstrap: Remove uses of 'git submodule' and 'sync-with-usptream'.
* .gitmodules: Remove.

Merge branch 'master' into core-updates

Conflicts:
	distro/packages/multiprecision.scm

build: Have `bootstrap' run all the necessary steps.

* bootstrap: Always run "git submodule init".  Run
  `nix/sync-with-upstream'.
  Reported by Nikita Karetnikov and Andreas Enge.

Merge branch 'nix-integration'

Conflicts:
	tests/guix-package.sh

build: Add `bootstrap' and `sync-with-upstream' scripts.

* bootstrap, nix/sync-with-upstream: New files.
* Makefile.am (EXTRA_DIST): Add `bootstrap'.
* daemon.am (EXTRA_DIST): Add `nix/sync-with-upstream'.