DB Credentials leak in upgrade_unattended.php

Retrieve credentials from Mantis system configuration instead of
accepting them from POST parameters.

This issue was reported by Matthias Karlsson (http://mathiaskarlsson.me)
as part of Offensive Security's bug bounty program [1].

Paul Richards' original patch was modified to align the code with master
branch to (basically replacing DIRECTORY_SEPARATOR by '/') to facilitate
porting.

Fixes #17877

[1] http://www.offensive-security.com/bug-bounty-program/

Signed-off-by: Damien Regad <dregad@mantisbt.org>