kern_select(9) copies fd_set in and out of userspace in quantities of

kern_select(9) copies fd_set in and out of userspace in quantities of
longs. Since 32bit processes longs are 4 bytes, 64bit kernel may copy in
or out 4 bytes more then the process expected.

Calculate the amount of bytes to copy taking into account size of fd_set
for the current process ABI.

Diagnosed and tested by:	Peter Jeremy <peterjeremy acm org>
Reviewed by:	jhb
MFC after:	1 week