master cron Problem: calling randombytes_close with libsodium can crash …
* add opt-out for randombytes_close
Problem: randombytes_close is either a no-op or unsafe when a Context is running.
Unfortunately, there does not appear to be a single always correct choice,
so let builders pick between two not-great options.
Opting out can leak an FD on /dev/urandom which may need to be closed explicitly.
However, with the default behavior,
using multiple contexts with CURVE can crash with no application-level workaround available.
randombytes_close is not threadsafe and calling it while still in use by a Context can cause a crash.
For implementations using /dev/[u]random, this can leave up to one leftover FD per process.
The libsodium docs suggest that this function rarely needs to be called explicitly.