Add ability to specify keyctl scope.

[jmczerk]

Resolves the issue described in this comment, enabling keyctl backend to work in Linux by allowing a user to specify a keyctl scope.

Current behavior:

sh-4.2$ ./aws-vault login --debug --backend keyctl default
2023/04/06 22:07:45 aws-vault dev
2023/04/06 22:07:45 Using prompt driver: terminal
2023/04/06 22:07:45 [keyring] Considering backends: [keyctl]
2023/04/06 22:07:45 [keyring] Failed backend keyctl: accessing "" keyring failed: unknown scope ""
aws-vault: error: Specified keyring backend not available, try --help

With new --keyctl-scope flag (SUCCESS):

sh-4.2$ ./aws-vault login --debug --backend keyctl --keyctl-scope user default
2023/04/06 22:08:22 aws-vault dev
2023/04/06 22:08:22 Using prompt driver: terminal
2023/04/06 22:08:22 [keyring] Considering backends: [keyctl]
2023/04/06 22:08:22 Loading config file /home/ec2-user/.aws/config
2023/04/06 22:08:22 Parsing config file /home/ec2-user/.aws/config
2023/04/06 22:08:22 profile default: using credential process
2023/04/06 22:08:22 Re-using cached credentials ****************UNZD from credential_process, expires in 14m54.11233509s
2023/04/06 22:08:22 Requesting a signin token for session expiring in 14m54.056822686s

With new AWS_VAULT_KEYCTL_SCOPE environment variable (SUCCESS):

sh-4.2$ AWS_VAULT_KEYCTL_SCOPE=user ./aws-vault login --debug --backend keyctl default
2023/04/06 22:10:07 aws-vault dev
2023/04/06 22:10:07 Using prompt driver: terminal
2023/04/06 22:10:07 [keyring] Considering backends: [keyctl]
2023/04/06 22:10:07 Loading config file /home/ec2-user/.aws/config
2023/04/06 22:10:07 Parsing config file /home/ec2-user/.aws/config
2023/04/06 22:10:07 profile default: using credential process
2023/04/06 22:10:07 Re-using cached credentials ****************UNZD from credential_process, expires in 13m9.003402192s
2023/04/06 22:10:08 Requesting a signin token for session expiring in 13m8.951533886s

[jynolen]

Can confirm if can fix issues with keyctl.
Please consider mrege before next release.

Thank you

[ms-zlab]

need this fix, when will it be merged?