[MOD] STR #4219: Scheduler does not validate client-provided UTF8 strings

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[STR New]

libdbus now performs strict checking for UTF8 validity in string
parameters. This causes an assertion failure when a required attribute is
not valid UTF-8.

I think the scheduler ought to validate UTF8 strings provided by clients.

Example assertion failure caused by this:
  https://bugzilla.redhat.com/show_bug.cgi?id=863387

(Originally from STR #3984)

Link: http://www.cups.org/str.php?L4219
Version: 1.6.1