Shared memory: user-definable prefix

[spixi]

Some sandbox applications like Ubuntu Snaps restrict access to /dev/shm/. There is already a workaround for the tbs server to use sockets instead of shared memory (--tbs_used_shared_mem=false), but the --tbs_server_semaphore option only adds an infix, but not a prefix to the name of the semaphore. It should be possible to provide a prefix like snap.argentumage. to the shared memory named pipes and semaphores by command-line option.