modules/xcap_server: fixed fetching of an element (GET with XPath)

- This is a mandatory part of XCAP and DELETE/PUT of elements (using XPath)
  is supported in xcap_server.
- The code to get a node from a document was all in there but never called.

(cherry picked from commit 6b8b8a050e4490302d1f1f940a7fda95784cce46)
tm: fix possible segfault in cancel_branch()
modules/rls: Fix memory leak in rls
- Leak would occur in two error cases
- Also improved diagnostics to display uri on various failures
(cherry picked from commit 52ac54133f70776a2dfe54e1789de8b1cae02a05)
uac: reset registration authentication flag

- reported by  rene montilva (fs#352)

(cherry picked from commit bc66a385e8a282638c8d5cfa566f971c62cd341b)