key.cpp: fail with a friendlier message on missing ssl EC support

Previously if bitcoind is linked with an OpenSSL which is compiled
without EC support, this is seen as an assertion failure "pKey !=
NULL" at key.cpp:134, which occurs after several seconds. It is an
esoteric piece of knowledge to interpret this as "oops, I linked
with the wrong OpenSSL", and because of the delay it may not even
be noticed.

The new output is

: OpenSSL appears to lack support for elliptic curve cryptography. For
more information, visit
https://en.bitcoin.it/wiki/OpenSSL_and_EC_Libraries
: Initialization sanity check failed. Bitcoin Core is shutting down.

which occurs immediately after attempted startup.

This also blocks in an InitSanityCheck() function which currently only
checks for EC support but should eventually do more. See #4081.

src/key.cpp

@@ -631,3 +631,15 @@ bool CExtPubKey::Derive(CExtPubKey &out, unsigned int nChild) const {
     out.nChild = nChild;
     return pubkey.Derive(out.pubkey, out.vchChainCode, nChild, vchChainCode);
 }
+
+bool ECC_InitSanityCheck() {
+    EC_KEY *pkey = EC_KEY_new_by_curve_name(NID_secp256k1);
+    if(pkey == NULL)
+        return false;
+    EC_KEY_free(pkey);
+
+    // TODO Is there more EC functionality that could be missing?
+    return true;
+}
+
+