Fix shared memory crashes in TCP forks

[DL6ER]

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What does this PR aim to accomplish?:

Fix #1478

How does this PR accomplish the above?:

Two changes: (1) Don't check SHM ownership on every SHM interaction but only in the danger zone (when reopening during resizing) and (2) Don't remap the global setting object but only read the few bytes we need to determine whether the PID has changed (or not).

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None

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[yubiuser]

Confirmed working in #1478