(We do this unconditionally, since we still need it for tortls.c)

Notably, there's a test to make sure that it round-trips with
OpenSSL, if OpenSSL is enabled.

We only ever need this to get us a DH ephemeral key object,
so make a function that does just that.

This is comparatively straightforward too, except for a couple of
twists:

   * For as long as we're building with two crypto libraries, we
     want to seed _both_ their RNGs, and use _both_ their RNGs to
     improve the output of crypto_strongest_rand()

   * The NSS prng will sometimes refuse to generate huge outputs.
     When it does, we stretch the output with SHAKE.  We only need
     this for the tests.

This was a fairly straightforward port, once I realized which layer
I should be calling into.

Part of 26979.

Closes 26979.

To avoid future duplicates.

Part of 26979.

Part of 26979.

Fixes bug 26986; bugfix on master.

Part of 26979.

That's what git does for tor.

Part of 26979.