Drop support for openssl 0.9.7; complain about 0.9.8
OpenSSL 0.9.7 is old and unsupported by the openssl team.
OpenSSL 0.9.8 doesn't have the improved crypto implementations of 1.0.0 and 1.0.1.
Removing support for the former lets us remove our sha256 implementation and a modest pile of backward compatibility code. Complaining about the latter will maybe encourage people who build from source to build with a better openssl.
See branch openssl_1_is_best in my public repo.
Before merging this, we should confirm that we don't care about Tor 0.2.4 dropping support for any distro that's been trying to patch 0.9.7; do they exist?