MEDIUM: pipes: Have one pool of free pipes per thread group

MEDIUM: pipes: Have one pool of free pipes per thread group

The pipes used for splicing are kept in a shared pool of free pipes, so
that they can be reused by any thread. However, with per-thread-group
FD tables, a pipe created by one group only exists in that group's
kernel table, so its FDs must never be handed over to a thread of
another group.
To fix that, replace the global list of free pipes with one list per
thread group if the fd tables are not shared across thread groups.