_krb5_load_plugins: Windows naming rules
Windows plugins must be loaded from the same directory as the
heimdal.dll in order to ensure that "DLL Hell" is not revisited.
Since plugins cannot be grouped in a directory by "module" name
the plugins must have a name of the form:
plugin_<module>_<name>.dll
Construct the "plugin_<module>_" prefix string dynamically and
test for both the prefix and the ".dll" extension when building
the list of plugins to load.
Change-Id: I0ff7bf7287f8fb38061c226f0844d7d0f1fb59ec