Netconf Confirmed Commit Capability

[olofhagsand]

Implement Netconf Confirmed Commit Capability according to RFC 6241

The current code only has a placeholder for the cancel-commit operation in backend_client.c, nothing else is implemented, except for the generic mechanism given by ietf-netconf@2011-06-01.yang

Some comments/hints:

• A timeout can be set using clixon_event_reg_timeout().
• If persistence is used, it is not enough to save the previous config state in server memory. One way to achieve consistency is to save restoration state using a temporary datastore. The datastore API in clixon_datastore.h works on any db and could be used for this purpose. Eg xmldb_copy("running", "persistent-db")
• Reverting state on timeout/cancel could be made by copying back from persistent-db to candidate and then issuing a commit.

[olofhagsand]

FIxed privileges drop and check running lock completes all unknown issues.
Thanks @pheller for the major part.