[review] libpq: Support TLSv1.1+ (was: fe-secure.c and SSL/TLS)

I applied both libpq.tls11plus.diff and the related
psql.conninfo.tlsver.diff patch to postgresql git head.

Source review:

The source changes are pretty tiny. Although I think the change
from TLSv1_method to SSLv23_method is correct, the comment is not
quite correct:

> * SSLv23_method() is only method that negotiates
> * higher protocol versions.  Rest of the methods
> * allow only one specific TLS version.

As I understand it (backed up by a quick glance through the openssl
source), the TLSv1_method, TLSv1_1_method, and TLSv1_2_method will
all advertise the corresponding protocol version to the peer, meaning
that in practice they will negotiate *up to* that TLS version, but
will still negotiate down to SSLv3. So, using TLSv1_2_method would
give the right behavior when compiled against a recent openssl.
However, someday when TLSv1.3 (or 2.0) appears, presumably the
SSLv23_method will be extended to include it but TLSv1_2_method
would have to be changed to TLSv1_3_method. Therefore using
SSLv23_method and disabling older protocol versions with
SSL_CTX_set_options() should have the desired behavior even in
future versions. (And it doesn't require autoconf to probe the
openssl version.)


Testing:

I built the patched postgresql against a handful of openssl versions:
1.0.1 (netbsd, x86-64, supports TLSv1.1); Git head aka 1.0.1f++
(osx, x86-32, supports TLSv1.2), and 0.9.8y (osx, x86-32, supports
TLSv1.0). They all built cleanly and passed 'make check'. I also
built 'contrib' and installed the sslinfo extension. I connected
between each pair of versions (with psql) and saw that the connection
negotiated the highest protocol version supported by both ends and
a corresponding ciphersuite. /conninfo and the sslinfo extension
agreed on the protocol version and ciphersuite in use.

Things I didn't test:

Client certificates, restricted sets of ciphersuites, MITM
protocol-downgrade attacks, non-x86 architectures, or 1.0.0* versions
of openssl.