pgsql: Support specifying filename for SSL certificate, key, root

Log Message:
-----------
Support specifying filename for SSL certificate, key, root certificate store
and certificate revokation list by using connection parameters or environment
variables.

Original patch by Mark Woodward, heavily reworked by Alvaro Herrera and
Magnus Hagander.

Modified Files:
--------------
pgsql/doc/src/sgml:
libpq.sgml (r1.273 -> r1.274)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.273&r2=1.274)
pgsql/src/interfaces/libpq:
fe-connect.c (r1.370 -> r1.371)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c?r1=1.370&r2=1.371)
fe-secure.c (r1.113 -> r1.114)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-secure.c?r1=1.113&r2=1.114)
libpq-int.h (r1.137 -> r1.138)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-int.h?r1=1.137&r2=1.138)