Use config API to access allow_browser_cache

Until now $g_allow_browser_cache config was handled in a non-standard
way in in http_caching_headers(), i.e. by referencing it as a global
variable instead of retrieving its value with config_get_global().

Moreover, the setting was commented out in config_default_inc.php
(i.e. not defined) instead of being set to OFF.

This follows up on discussion in PR
https://github.com/mantisbt/mantisbt/pull/1925

Fixes 0033482