[dim] dim-0.4.7-mv4 export bug

[Philippe Bergheaud]

$ dim mklib file:/home/felix/selflib /home/felix/selflib
$ export DIMLIB=/home/felix/selflib
$ dim mkuser
$ dim mkitem rr
$ dim mount rr .
$ dim export -n
rr-0-pb1        # good
$ dim export -n -M
rr-1            # good
$ dim export -n -M -B 37
rr-37           # good
$ dim export -n -B 37
rr-0--37        # bad