Auto-regenerate expired self-signed certificates

[lzwind]

Summary

• _httpTLSStart checked whether certificate files exist on disk via access() but never verified whether they have expired. When an auto-generated self-signed certificate expired, CUPS continued using it instead of creating a new one, breaking HTTPS access.
• Added a cupsGetCredentialsExpiration() check so that expired certificates are treated as missing, triggering the existing auto-create code path.
• Applied to both tls-openssl.c and tls-gnutls.c backends.

Fixes: #1519

Test plan

• Create a self-signed cert, manually set its expiration to the past, and verify CUPS regenerates it on next startup
• Verify that valid (non-expired) certificates are still used normally
• Test with both OpenSSL and GnuTLS backends

[abubakarsabir924-cell]

Hi @lzwind,
Just checking in — are you still working on this PR? I noticed it's been open for about a month without updates.
If you're busy or have moved on, I'd be happy to help finish it up (completing the test plan, or refining the approach if needed). Let me know either way — happy to collaborate or step back, whatever works.
Thanks!