Preparations for dropping the old plugin api.

Preparations for dropping the old plugin api.
Introduce a new init routine for plugins,
which does not take the plugin api table as an
argument and allows etheral to distinguish
between plugins using the old and the new api.

Update README.plugins accordingly

Change all g_warnings() in epan/plugins.c to report_failue().
On windows we do not have a log console open while
loading the plugins, because a log console cannot be opened before the prefs have been read. Thus g_warnings()
does not work for reporting problems with plugins.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@13596 f5534014-38df-0310-8fa8-9805f1628bb7