Make ssh-agent startup arguments configurable
This code
doesn't provide any options to run ssh-agent from gnome-keyring, and ssh-agent doesn't accept any config file.
Providing some arguments can be useful in situations similar to CVE-2023-38408 (https://blog.qualys.com/vulnerabilities-threat-research/2023/07/19/cve-2023-38408-remote-code-execution-in-opensshs-forwarded-ssh-agent) where disabling/limiting PKCS#11 modules via command line was one of possible workarounds.