rng: split initialization in preinit and init

Merged Nikos Mavrogiannopoulos requested to merge tmp-init-random-generator-on-first-call into master

This makes gnutls to initialize its random generator on the first call to gnutls_rnd(). That prevents blocking due to getrandom() on a constructor; that change allows to use gnutls-linked applications even in early boot in systems where getrandom() blocks waiting for entropy.

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