util: Do not leak dup-ed file descriptor

When going daemon criu services ask cr_daemon() to keep
some descriptors open and the routine dup2-s them. The
problem is that the original descriptor is left where it
was and is leaked.

travis-ci: success for util: Do not leak dup-ed file descriptor
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>