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. It is an esoteric piece of knowledge to
interpret this as "oops, I linked with the wrong OpenSSL".

The new output is

  Fatal error: OpenSSL appears to lack support for curve secp256k1.

The reason for using abort() rather than, say, exit(EXIT_FAILURE)
is that a clean shutdown calls the destructor for CCheckQueue, which
is not fully initialized at this time. This causes a messy Boost
assertion failure to fire. This keeps the failure local.