#126 xmlstarlet val: Consistent message order

[Ernst de Haan]

Currently the order of the messages output from xmlstarlet val is not consistent. Please ensure the order is always the same.

This is important for unit tests, as they may expect the output in a specific order.

Steps to reproduce:
1. Have an XML file a.xml with an tag <unknown_tag/>.
2. Have an XML Schema definition file b.xsd that does not define the unknown_tag.
3. Run xmlstarlet validate --xsd b.xsd --err a.xml many times (e.g. 20 times).

Expected results:
1. On step 3, the output is consistently the same, e.g.:

a.xml:2.13: Element 'unknown_tag': No matching global declaration available for the validation root.
a.xml - invalid

Actual results:
1. Many times, the output is indeed the same. However, often the output lines are swapped, so then the output is:
a.xml - invalid a.xml:2.13: Element 'unknown_tag': No matching global declaration available for the validation root.