tools: fix segfault when invalid identity given

Giving no or invalid identity to dnc, dsh or dvpn was giving a segfault.
They now print an error message and exit with a non-zero status
correctly. dhtnet-crtmgr was also suffering segfault when no -o option,
even for --help or --version options. This is now fixed.

Change-Id: I7871db9ab73205c9ad024260d7687cc20ae1a983