Check prefixes and local names carefully, including unicode character ranges

PLX characters in local names are not validated in this changeset.
Provide regression tests for turtle's pname checking routine
Also validate PLX characters in raptor_turtle_is_legal_turtle_qname()
Fix, make sure all comparisons in xmbtowc() happen in unsigned int space
Fix up regression test, the local escape will be stripped off, so ...

... don't expect that in the output
Exclude turtle parse regression with illegal unicode character.

Our turtle parser allows more characters than the official specs.
We are extra-careful not to produce them in the turtle serializer but
accepting them as input seems to do no harm.