creating a new segment: use "xb" mode, fixes #2099
creating a new segment: use "xb" mode, fixes #2099

"ab" seems to make no sense here (if there is already a (crap, but non-empty) segment file,
we would write a MAGIC right into the middle of the resulting file) and cause #2099.

RonnyPfannschmidt/borg,edgimar/borg,edgewood/borg,RonnyPfannschmidt/borg,raxenak/borg,edgewood/borg,RonnyPfannschmidt/borg,raxenak/borg,RonnyPfannschmidt/borg,raxenak/borg,edgimar/borg,raxenak/borg,edgimar/borg,RonnyPfannschmidt/borg,edgewood/borg,edgewood/borg,edgimar/borg