Solution: restrict it only to the original issue #2632, Tweetnacl on
*NIX when using /dev/urandom, ie: without the new Linux getrandom()
syscall.

Existing applications might use atexit to register cleanup functions
(like CZMQ does), and the current change as-is imposes an ordering
that did not exist before - the context MUST be created BEFORE
registering the cleanup with atexit. This is a backward incompatible
change that is reported to cause aborts in some applications.

Although libsodium's documentation says that its initialisation APIs
is not thread-safe, nobody has ever reported an issue with it, so
avoiding the global init/deinit in the libsodium case is the less
risky option we have.

Tweetnacl users on Windows and on Linux with getrandom (glibc 2.25 and
Linux kernel 3.17) are not affected by the original issue.

Fixes #2991