Cookie security reform:
- fix handling of persistent logins while addressing problems of last commits
- increase usage of try/throw to be able to distinguish exceptions
- fix handling of LoginTimeout 0 in cryptographic expiration
- use [ad_conn behind_secure_proxy_p] on more occasions, where
security::secure_conn_p is used (maybe fold these together in the future)
- new private proc security::log to ease debugging of cookie management
- further improved documentation