Add case insensitive option to config file

[oxr463]

STATIC int verify_user(jwt_t * jwt, const char *username)
{
    const char *upn = jwt_get_grant(jwt, "upn");
    return (strcmp(upn, username) == 0) ? EXIT_SUCCESS : EXIT_FAILURE;
}

We are accepting input from two sources without normalizing either. I.E. if John Doe was in AAD as [email protected] he would have to auth via a service with Jdoe instead of being able to use jdoe. The strcmp will fail every time due to this even though the user exists and is valid. Will push a PR to fix.

Source: CyberNinjas/pam_aad#55