OpenSSL support

[xest]

Add ONION_USE_OPENSSL option to use OpenSSL instead of GnuTLS and GCrypt(default: false).
Tested on Ubuntu 16.10, OpenSSL 1.0.2g.

[davidmoreno]

Hi! Thanks a lot for the patch, but although it makes the trick, I dont like too much that it actually do a half wrapper for openssl using gnutls functions (particularly the use of the headers with the defines). It hides whats really going on.

I would prefer a openssl.c and a gnutls.c that implement the basic functions (basically whats on the defines and the extra headers), and make https.c call the proper implementation.

[xest]

Hi! I know what you said.
Actually I added random-openssl.c and https-openssl.c at first, but the differences between GnuTLS version and OpenSSL version were not too much.
But if you don't like this way, I'll do it again a few weeks after.