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.
Add support for globally defined forced $CFG

There are some additional $CFG variables that the mdk user may want to always
set in all config.php created. In my case, it is $CFG->divertallemailsto to
prevent accidental mails sent out from my local installations.

The patch adds support for the new mdk-config items grouped under `forceCfg`.
If the setting forceCfg.foobar is found, it's value will be put into
$CFG->foobar right after the installation process.
Merge branch 'forcecfg'