libfido2 debug output is written to stderr

[mlelstv]

When you use the debug option, libfido2 is initialized with FIDO_DEBUG, which sends debug output to stderr by default. This is impractical, debug output should go wherever the PAM debug output is sent to.

This patch configures a log handler in libfido2 that redirects libfido2 debug output accordingly.

pam-u2f.diff.txt

[LDVG]

Thank you for the patch!

While I would like to apply this, I'm a bit concerned at what happens when the application calls pam_end() and pam-u2f is unloaded. After this, if any other part of the program calls into libfido2, its log handler could be pointing to an unloaded function. To handle this safely, I believe we'd need a way of restoring libfido2's internal log handler (requiring changes to libfido2). Until that is possible, I think this will have to wait.