Fix a TOCTOU case when opening the debug_file
Fix a TOCTOU case when opening the debug_file

This also unifies the code between pam-u2f and yubico-pam which means
removing the O_CREAT flag here. This is however in line with the README
and the behaviour of pam-u2f. Also, the previous code did lstat on the
path before and if it did not exist it would not have moved on to the
open() anyways.

eworm-de/yubico-pam,Yubico/yubico-pam,Yubico/yubico-pam,eworm-de/yubico-pam,Yubico/yubico-pam,eworm-de/yubico-pam