Switch license to GPLv3

According to the decision of the GRNET Board of Directors,
switch license to GPLv3.

This commit will be propagated to the release
and master branches based on git flow, and the next
release will be licensed as GPLv3.
Fix libxsegmap generation

Also remove obsolete directories.
xseg: Change xseg_join() interface

Make pointers to segment name, segment type and peer type
pointers to constant.