MEDIUM: fd/poller: turn polled_mask to group-local IDs

MEDIUM: fd/poller: turn polled_mask to group-local IDs

This changes the signification of each bit in the polled_mask so that
now each bit represents a local thread ID for the current group instead
of a global thread ID. As such, all tests now apply to ltid_bit instead
of tid_bit.

No particular check was made to verify that the FD's tgid matches the
current one because there should be no case where this is not true. A
check was added in epoll's __fd_clo() to confirm it never differs unless
expected (soft stop under thread isolation, or master in starting mode
going to exec mode), but that doesn't prevent from doing the job: it
only consists in checking in the group's threads those that are still
polling this FD and to remove them.

Some atomic loads were added at the various locations, and most repetitive
references to polled_mask[fd].xx were turned to a local copy instead making
the code much more clear.