Milter-greylist

> Is there any merit to keep the select() code along with the poll() flavor?

I thought about this as well, but I was not brave enough to propose this one.
(minimalistic change approach)

The poll() is superior than select().
Actually select() is archaic with its file descriptor number restriction,
its use should be avoided as much as possible.

> I would rather replace it all (like below). What is your opinion?

In principle OK, but I couldn't apply your code since all tabs
are replaced with spaces in the inline text of this patch.
Cut & paste leads to invalid patch for me.
May be in format of attachment instead of inline?

The second occurrence of POLLHUP should be POLLNVAL, just
reading the code of the patch.

You may optimise out the check for select() from the configure script as well.