fix atexit when it is called from an atexit handler

The old code accepted atexit handlers after exit, but did not run them
reliably. C11 seems to explicitly allow atexit to fail (and report
such failure) in this case, but this situation can easily come up in
C++ if a destructor has a local static object with a destructor so it
should be handled.

Note that the memory usage can grow linearly with the overall number
of registered atexit handlers instead of with the worst case list
length. (This only matters if atexit handlers keep registering atexit
handlers which should not happen in practice).

Commit message/rationale based on text by Szabolcs Nagy.

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