(ef_open): Failed to reset empfile member fd on failure, and leaked

(ef_open): Failed to reset empfile member fd on failure, and leaked
file descriptors.

(ef_flush, ef_close): Rely on negative fd instead of null cache to
detect unopened file.  This is more logical and consistent with the
use of cache by the new tables that are not file-backed (rev. 1.22).
Use CANT_HAPPEN().

(ef_close): Failed to report cash flush failure.

(ef_close): Clear all EFF_OPEN flags.  Should have been done in
rev. 1.27.