Make it impossible to truncate or overflow the connection description

It's hard to predict the length of formatted output, so we'd better
notice (and abort) if the description is truncated. Incidentally,
mkdtemp() does this for us in the shared memory branch, but do an
explicit check there as well for consistency, and get rid of the wrongly
parametrized strncat() risking a buffer overflow (CONNECTION_DESCRIPTION
is not the length of the source "/qb").

Similar truncation checks should be added to qb_ipcs_{shm,us}_connect()
where they build the request/response names, and possibly to other
places using snprintf().