winsync-migrate: Convert entity names to posix friendly strings

During the migration from winsync replicated users to their
trusted identities, memberships are being preserved. However,
trusted users are external and as such cannot be added as
direct members to the IPA entities. External groups which
encapsulate the migrated users are added as members to those
entities instead.

The name of the external group is generated from the type
of the entity and its name. However, the entity's name can
contain characters which are invalid for use in the group
name.

Adds a helper function to convert a given string to a string
which would be valid for such use and leverages it in the
winsync-migrate tool.

https://fedorahosted.org/freeipa/ticket/5319

Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>

ipapython/ipautil.py

@@ -1330,6 +1330,29 @@ def restore_hostname(statestore):
         except CalledProcessError as e:
             print("Failed to set this machine hostname back to %s: %s" % (old_hostname, str(e)), file=sys.stderr)
 
+def posixify(string):
+    """
+    Convert a string to a more strict alpha-numeric representation.
+
+    - Alpha-numeric, underscore, dot and dash characters are accepted
+    - Space is converted to underscore
+    - Other characters are omitted
+    - Leading dash is stripped
+
+    Note: This mapping is not one-to-one and may map different input to the
+    same result. When using posixify, make sure the you do not map two different
+    entities to one unintentionally.
+    """
+
+    def valid_char(char):
+        return char.isalnum() or char in ('_', '.', '-')
+
+    # First replace space characters
+    replaced = string.replace(' ','_')
+    omitted = ''.join(filter(valid_char, replaced))
+
+    # Leading dash is not allowed
+    return omitted.lstrip('-')
 
 @contextmanager
 def private_ccache(path=None):