Make Certbot runnable on Windows (#6296)

* Add and use a compatibility layer to allow certbot to be run on windows.

* Fix path comparison

* Corrections on compat and util for tests

* Less intrusive way to parse prefix in webroot plugin working for both linux and windows.

* Disable pylint import-error for some optional imports in compat.py

* Ensure path is normalized before prefixes are generated in webroot plugin

* Same prefixes in linux and windows, in fact root path is not needed in webroot plugin

* Check that user has administrative rights before continuing on windows (necessary for symlink creation)

* More straightforward way to test administrative rights on windows

* Try to resolve import error in travis ci

* OK. We go for full introspection to trick the ci.

* Move the administrative rights control to the certbot entrypoint

* Add comment for a really non trivial code.

* Allow some commands to be run on a shell without admin rights

* Avoid races conditions on windows for lock files

* Add sphinx doc to the compat functions.

* Remove irrelevant Windows error in the lock mechanism.

* Some corrections on compat

certbot/plugins/util.py

@@ -9,18 +9,19 @@ logger = logging.getLogger(__name__)
 def get_prefixes(path):
     """Retrieves all possible path prefixes of a path, in descending order
     of length. For instance,
-        /a/b/c/ => ['/a/b/c/', '/a/b/c', '/a/b', '/a', '/']
+        (linux) /a/b/c returns ['/a/b/c', '/a/b', '/a', '/']
+        (windows) C:\\a\\b\\c returns ['C:\\a\\b\\c', 'C:\\a\\b', 'C:\\a', 'C:']
     :param str path: the path to break into prefixes
 
     :returns: all possible path prefixes of given path in descending order
     :rtype: `list` of `str`
     """
-    prefix = path
+    prefix = os.path.normpath(path)
     prefixes = []
     while len(prefix) > 0:
         prefixes.append(prefix)
         prefix, _ = os.path.split(prefix)
-        # break once we hit '/'
+        # break once we hit the root path
         if prefix == prefixes[-1]:
             break
     return prefixes