store: Wait for the server to be done sending output.

* guix/store.scm (current-build-output-port): New variable.
  (process-stderr): Add docstring.  Always return #f, except upon
  %STDERR-LAST.  Upon %STDERR-NEXT, write to
  `current-build-output-port', not `current-error-port'.
  (set-build-options): Loop until `process-stderr' returns true.
  (define-operation): Likewise.
  (build-derivations): Update docstring to mention that it's
  synchronous.