Fixes prompted by review.

* bzrlib/tests/test_plugins.py:
(TestEnvPluginPath.test_duplicates_are_removed): Check that paths
are clean.
(TestEnvPluginPath.test_bogus_references): Check robustness.

* bzrlib/plugin.py:
(get_site_plugin_path): Windows has no idea of a 'site' directory.
(get_standard_plugins_path): Simplified.

* bzrlib/help_topics/en/configuration.txt: 
Fix typos and bad grammar.