Respect the 'path' config when setting $CFG->sessioncookiepath in config.php

Setups using the 'path' config ended with invalid $CFG->sessioncookiepath set
in config.php (always expecting the moodle is in the root). Moodle core itself
has checks for this so the value set in config.php was ignored in that case.
Merge pull request #85 from mudrd8mz/fix-sessioncookiepath

Respect the 'path' config when setting $CFG->sessioncookiepath