xseg: Change posixfd directory permissions

Change '/run/shm/posixfd' directory permissions from '01755'
to '01777'. An ordinary user can now create/unlink his named
pipes.
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: Fix condition when checking for valid fd
xseg: Change xseg_join() interface

Make pointers to segment name, segment type and peer type
pointers to constant.
xseg: Fix uninitialized return value
xseg: Support X_FLUSH operation
xseg: Support X_TRUNCATE operation
xseg: Fix bug in return value
xseg_posixfd: Fix buffer overflow in posixfd_init_signal_desc

The length passed in hexlify() must be equal to the half
of the POSIXFD_FILENAME_LEN because hexlify() computes hashes
in pairs based on the address of the signal descriptor pointer.

Also double the size of the name of the signal descriptor.
Add trace info to xseg_request