Change private key file permissions

[zedt3ster]

Issue Addressed

The account_manager generates private keys and stores them with unnecessary extensive permissions. Indeed, private keys are readable by any user of the system (i.e. -rw-r--r-- or 0644 permission bits).

Proposed Changes

• Import libc crate for permission bits
• Change permissions on private key file generation to -rw------- (i.e. 0600)

Additional Info

The validator client is about to be refactored, but thought I'd submit this now anyway :)

[paulhauner]

@mzee I think these tests will pass if you merge in master :)