This code was originally written for non-reentrant crypt(3).
In 5f521d7ba72, a thread-safe crypt_r(3) was introduced. However,
it looks like the DES implementation is still not re-entrant;
routines like setup_salt() or des_setkey() still use global
Instead of something drastic, eg removing DES support altogether,
just mark those variables as thread-local.
Given that this only applies to DES, I think the impact is minimal.
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