Make sentinel value configurable

There are OSs that always return the lowest free value.
The algorithm in `lazy_init` always avoids keys with the
sentinel value.
In affected OSs, this means that each call to `lazy_init`
will always request two keys from the OS and returns/frees
the first one (with sentinel value) immediately afterwards.

By making the sentinel value configurable, affected OSs can
use a different value than zero to prevent this performance
issue.

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