Doesn't compile against php7

[dfuhry]

In order to compile against php7, in session_pgsql.c instances of smart_str need to be changed to smart_string (and corresponding method calls smart_str__(...) to smart_string__(...)), and functions like zend_register_auto_global which previously accepted C-string parameters now require zend_string parameters.
There well may be more problems in addition. A useful extension porting reference is at: https://wiki.php.net/phpng-upgrading