add support for persistent srk

[novag]

• Persisting the SRK allows the creation and use of individual keys without specifying the owner password

Example:

1. Set an owner password: tpm2_changeauth -c owner myPass
2. Create SSH key and save SRK to handle ID 0x81000002: ssh-tpm-keygen -o -s 0x81000002
3. Create another SSH key without entering the owner password: ssh-tpm-keygen -s 0x81000002
4. Start the SSH agent without entering the owner password: /ssh-tpm-agent -s 0x81000002 -l /var/tmp/tpm.sock

Based on #37