DKG CLI write keys with 600

[gakonst]

5.1 [dkg-cli] Private keys written with read permissions Severity: medium
5.1.1 Description
The private keys generated by the dkg new utility are written with 644 permissions, allowing them to be read by all users of the system. This may expose private keys to unauthorized users.
5.1.2 Recommendation
Permissions should preferably be set to 600, to disallow read access from other users.