Change select() so that only as many words as necessary for the 'nfds'

Change select() so that only as many words as necessary for the 'nfds'
argument from the fd bitmasks are copied from and back to userspace. This
solves an ABI dependency on OPEN_MAX. If nfds is too big for the current
OPEN_MAX, select() fails (but that's relatively easy to fix by 'just'
recompiling the system and not the application binaries), but if it's
smaller, binaries keep working.