Added config_set_global() to config API for plugin system.
Implemented plugin configuration handling, with default and explicit configuration options.
Minor sanity check in plugin API.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@4805 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9