BUGFIX lib/cnl: stdio handlers must have all vars closed in local scope

Signed-off-by: Oleg Verych <olecom@gmail.com>
comments cleanup; delete `echo` from do

Signed-off-by: Oleg Verych <olecom@gmail.com>
l10n: change wording

Signed-off-by: Oleg Verych <olecom@gmail.com>
lib/cnl: add error handling in child; more stdio error handling

Signed-off-by: Oleg Verych <olecom@gmail.com>