pgsql: Fix libpq certificate validation for SSL connections.

Log Message:
-----------
Fix libpq certificate validation for SSL connections.

Add config parameter "sslverify" to control the verification. Default
is to do full verification.

Clean up some old SSL code that never really worked.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        libpq.sgml (r1.268 -> r1.269)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.268&r2=1.269)
        runtime.sgml (r1.419 -> r1.420)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml?r1=1.419&r2=1.420)
    pgsql/src/interfaces/libpq:
        fe-connect.c (r1.367 -> r1.368)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c?r1=1.367&r2=1.368)
        fe-secure.c (r1.106 -> r1.107)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-secure.c?r1=1.106&r2=1.107)
        libpq-int.h (r1.136 -> r1.137)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-int.h?r1=1.136&r2=1.137)