Avoid generating content-location header if no valid URIs

The oBIX server segfaults with following request:

	curl localhost//obix

The root cause lies in that when an error occurs there is a chance
that both response_uri and request_decoded_uri are NULL which will
result in passing a NULL parameter to FCGX_FPrintF and triggers
coredumps.

The content-location HTTP header should be skipped over in such case
Impose further control on requested URI

Aside from leading double slashes, no double slashes or any dot in
any position or leading whitespace characters are allowed in the
requested URI.
Enforce sanity checks on href in provided XML document on client side

If the root node of the provided XML document from oBIX client has a
href attribute, it must be a valid one.
Rename xml_href_is_valid to xml_is_valid_href

So as to confrom with with naming convention of other functions such
as xml_is_valid_doc
Merge pull request #74 from Qingtao-Cao/devel

Impose stricter control on requested URI on both server and client side