DEBUG: fd: add a counter of takeovers of an FD since it was last opened

DEBUG: fd: add a counter of takeovers of an FD since it was last opened

That's essentially in order to help with debugging strange cases like
the occasional epoll issues/races, by keeping a counter of how many
times an FD was taken over since last inserted. The room is available
so let's use it. If it's needed later, this patch can easily be reverted.
The counter is also reported in "show fd" as "tkov".