Implement the first of two features requested in the bug: when
encountering a broken .xml schema file, back out the changes in that
file and continue to parse other files.
This prevents a single broken .xml file from messing up GSettings for
everyone else.
Add a --strict option to get the old behaviour. Use this from the test
cases.
- can not extend schemas that already have paths
- can not form list of schemas that already have paths
- the path of a list schema, if given, must end with ':/'
