XPath match accepts unresolved namespace

[olofhagsand]

Clixon full namespace match, such as in a call to:

    xpath_first(x, nsc, xpath)

tolerates unresolved namespaces in xpath (ie that are not in nsc).
Forcing this check breaks some code and some tests.
A temporary compile-time constant XPATH_NS_ACCEPT_UNRESOLVED has been added as a work-around until the code and tests have been upgraded.