xlock: Add 'xlock_' prefix to {pack, unpack}_owner
xseg-tool: Fix recoverlocks functionality
xseg-tool: Fix verify functionality
xseg_posixfd: Add S_ISGID bit to fifo's directory

Add S_ISGID bit to fifo's directory. This way each fifo created under this
directory inherits the group id of the POSIXFD directory. This enables different
Archipelago components to signal each other independently from their effective
user id.