* auth: rotate session ID on all authenticated transitions (session fixation)
The PHP session ID was regenerated on explicit form login, reauth, logout and
password change, but NOT when access is (re)established from a remember-me
cookie, via HTTP authentication, or on self-registration auto-login (CWE-384),
so a fixed session ID could become authenticated.
Regenerate the session ID at those transitions too: in FreshRSS_Auth::init
when accessControl() first grants access (covers remember-me restore and HTTP
auth; skipped for auth_type 'none', which has no authentication boundary), and
in the self-registration auto-login path, mirroring the existing form-login
idiom. Also enable session.use_strict_mode before session_start as defence in
depth so an uninitialised, attacker-supplied session ID is not adopted.
* auth: avoid duplicate cookie during session rotation
* Code preferences, shorten some comments
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Co-authored-by: Inverle <inverle@proton.me>
Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>
