poll.2: Set fd to its bitwise complement to ignore

poll.2: Set fd to its bitwise complement to ignore

A valid file descriptor will be non-negative, i.e., 0 <= fd <= INT_MAX.
When all of the bits of a valid file descriptor are flipped, the result
is a negative value: ~0 = -1, ~1 = -2, ..., ~INT_MAX = INT_MIN. If all
of these bits are flipped again, then the file descriptor is recovered.

Negating the file descriptor does not quite achieve this, because there
will be no change for fd 0. (Likewise, negating INT_MIN would not cause
any change in value, but it is never a valid file descriptor.)

Link: https://bugzilla.kernel.org/show_bug.cgi?id=79411
Signed-off-by: David Ward <david.ward@gatech.edu>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>