fast copy of select's fd_set

copyfd.c

#define _XOPEN_SOURCE 700
#include <sys/select.h>

static void copyfdset(fd_set *d, fd_set *s, unsigned fd_max) {
        unsigned i;
        unsigned n = (fd_max + 1 + __NFDBITS - 1) / __NFDBITS;
        for(i = 0; i < n; i++) {
                d->fds_bits[i] = s->fds_bits[i];
        }
}