server.ctx is now allocated once for each server on first connection, then reused for subsequent connections.
server.ctx wasn't being cleared when the server was closed (as server.ssl_fd is), so this was causing
reconnection to fail until try_connect() got called.
This also removes the last uses of the CHK_* macros in ssl.h, so remove them.
Tokens that begin with two underscores __* or an underscore and an uppercase letter _X* are reserved,
so we should avoid those for our own include guards. The standard I'm settling on for foo.h is FOO_H_.
