Identify safe, common, useful openssl engines to enable by default

Ever since f0d4b3d1 (svn revision r5829) , all of our crypto acceleration is off by default, since we can't trust any given openssl engine to be secure, stable, and to run without crashing.

We should identify engines which it would be safe and useful to turn on by default, and have them be on-by-default. IMO the criteria should be:

• It needs to be pretty common for a user to have the requisite hardware but not know about it. IOW, anybody who has bought a special-purpose board can configure it themselves, but people with CPU or chipset support for acceleration are likely not to have thought about it.
• It needs to be really stable.
• It needs to be pretty well distributed.
• It needs to be using a recent version of openssl.
• It needs to make an actual improvement to Tor's performance or security.
• We need to be able to test it.

Good candidates to look at for a start IMO are aes-ni instructions.

We'll also maybe need a UI change to let people disable default engines and add extra ones.