file-systems: Spawn a Bournish REPL upon fsck failure.

file-systems: Spawn a Bournish REPL upon fsck failure.

Fixes <http://bugs.gnu.org/22588>.
Reported by Mark H Weaver <mhw@netris.org>.

* gnu/build/file-systems.scm (check-file-system): Pass
%BOURNISH-LANGUAGE as the argument to 'start-repl'.
* gnu/services.scm (activation-script): Add (guix build bournish).
* gnu/services/base.scm (file-system-shepherd-service)[imported-modules]:
Likewise.
* gnu/system/linux-container.scm (container-script): Likewise.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Likewise.