Refactor: libcrmcommon: Improve xml_find_x_0_schema_index.

• Lots of comments to explain how it works.

• Walk the list backwards, stopping on the first one in the major version series. This means the first one no longer has to be X.0.

• Require that known_schemas be non-NULL.

• Don't use the returned index to also mean we've found something since that means if the index we actually want to return is 0, the function will have to run every time.