Accept IdP EntityID to use when metadata contains more than one IdP.
Allow specifying attribute names to get given name, surname, and email address.
Allow specifying NameIDFormat to request.
Allow specifying whether to get username from a named attribute, or NameID.
Allow getting administrator state from attribute.
If LOG_FILE config option contained invalid file, pdns admin wouldn't start.
Also, it wasn't possible to log only to stdout/stderr correctly, because
setting the log file to /dev/stdout would double the logs messages as the pdns
admin is already logging to the stderr.
