Close all unwanted open file descriptors before executing commands and mail.
    
    There might be file descriptors open by things like pam_systemd that
    we do not want to keep open for the child process.
    
    Original idea by: Jakub Jelen <jjelen@redhat.com>
    Upstream commit: b2c8cbcef8c97b5a175d6e71995249b288707b0f