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.