Add ssl_library preset parameter

This allows querying the SSL implementation used on the server side.
It's analogous to using PQsslAttribute(conn, "library") in libpq.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>

src/backend/libpq/be-secure.c

@@ -38,6 +38,7 @@
 #include "storage/proc.h"
 
 
+char	   *ssl_library;
 char	   *ssl_cert_file;
 char	   *ssl_key_file;
 char	   *ssl_ca_file;