OPTIM: fd: make each thread group's polled_mask NUMA-local

OPTIM: fd: make each thread group's polled_mask NUMA-local

The polled_mask array was left aside the fdtab's NUMA placement:
its slices are not padded to the page size, so a page can be shared
between two groups, and calloc() zeroes it eagerly on the boot
thread when the allocation is served from the heap. Allocate it
with mmap() and pad the slices as the fdtab: nothing is copied
nor zeroed, the pages are born zeroed and each group's masks are
written by its own threads, bound before they first touch them, so
the lazy faulting places them on the right node by itself.