[PATCH] Enable SSL library detection via PQsslAttribute

Hello,

As part of the NSS work it came up [1] that clients don't have a good
way to ask libpq what SSL library it was compiled with, unless they
already have a connection pointer so that they can call
PQsslAttribute(conn, "library"). This poses a chicken-and-egg problem:
with the NSS proposal, the client may have to know which library is in
use before it can construct a proper connection string. For example, I
have a test suite that needs to set up an NSS database with
certificates before telling libpq to connect using that database.

The simplest proposal was to just allow PQsslAttribute() to take NULL
as the connection parameter when querying the "library" attribute, and
that's what I've done in this patch. In current versions of libpq, the
"library" attribute will always be NULL if you pass NULL as the
connection; a client that needs to know whether this new behavior is
present can look at the LIBPQ_HAS_SSL_LIBRARY_DETECTION feature macro.

If this looks good, I'm not sure how best to test it in the regression
suite. I see that libpq has an installcheck recipe that compiles a test
executable for URI parsing; should I add a simple test alongside that?

Thanks,
--Jacob

[1] https://www.postgresql.org/message-id/a1798e46d6d801344ebc93672c6947ef5297c8a0.camel%40vmware.com