useFakeSession during install on Standalone

[ufundo]

Use a fake session during Standalone install, because we might not have the real session handler yet (because the extension that provides it hasn't been installed),

Before

Standalone web installer crashes because of trying to start a session during the install, before standaloneusers extension is enabled to provide the session handler.

After

Install completes successfully.

Technical details

I tried @totten 's suggestion here #29362 (comment) but it didn't seem to work. Because the Civi-bootstrap phase forces recreating the CRM_Core_Config singleton, the fakeness is lost.

I was also keen possible session creation during install by any pesky CRM_Core_Session::setStatus calls -- this doesn't work for that case because it's inside the $loadFromDB case, and at the beginning of the install we're not loading from DB.

The alternative that did cover this case is to put a check inside CRM_Core_Session::singleton, but this seems quite aggressive. (Ie. #29488 )

After all this I'm thinking the initial approach of using default PHP session handler might be the neatest way. For one thing it keeps the Standalone specific code inside userSystem, which seems desirable.

[civibot]

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[artfulrobot]

Closing as #29352 merged.