Re: Supporting Windows SChannel as OpenSSL replacement

Here's a new version of the refactoring patch. I've fixed the issues
reported so far.

Upon looking closer at the SIGPIPE stuff in libpq, I realized that we
can remove this line from fe-secure-openssl.c:

> - /* We cannot use MSG_NOSIGNAL to block SIGPIPE when using SSL */
> - conn->sigpipe_flag = false;

That's because all the I/O now goes through our wrapper functions that
do the send/recv, and will use MSG_NOSIGNAL if it's available. That
avoids two syscalls per send. I haven't measured the performance impact
of that - it's probably negligible compared to doing encryption - but
it's still nice to avoid it.

This patch is just refactoring of existing code. It doesn't have any
user-visible changes; the libpq functions that expose OpenSSL stuff are
still intact. Doing something about those are the next phase of this
project.

Please review. I think this is in a good shape, ready for commit.

- Heikki