Emit BackendXID and check c_flag/b_flag properly

07d488cb96c703dd7c0ccf7049448c6291f1f0cf partially broke varnishlog,
since libvarnishapi did not properly check if both -c and -b were
given.

In addition, varnishd did not emit a tag at the start of a backend
transaction.  Emit BackendXID since that seems appropriate.

Fixes #1325