Robust handling of external command calls

If an external command like DelegationSignerSubmitCommand or NotifyCommand fails in a way that goes unnoticed for popen, currently OpenDNSSEC can't detect it.

Attached you will find a C code that tries to popen a perl script, which contains a syntax error. Neither popen nor fprintf detect the error, only at pclose a SIGPIPE signal is sent. This means the failure goes under the radar.