kern/126076: [headers] POLL_ERR and POLL_HUP are defined identically

[Petr Salinger]

> According to kib this is intentional. POLL_ERR is not implemented,
> so it is simply mapped to POLL_HUP for now.

It would be much better to reserve new value for either
POLL_ERR or POLL_HUP just now.

The current situation does not conform to POSIX-2008, see
http://www.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html

Petr