implement poll and read

No matter if we implement some other kind of dirty notification mechanism,
it feels right to have pwqr pollable for overcommit.

Documentation:
- drop the "in kernel unpark" method, it sucks
- migrate to using non blocking "read" for the probing method.
- document the pollability and how read works in the pwqr_create "manpage".

lib:
- implement epoll_create with flags. It requires a kernel supporting
  O_NONBLOCK/O_CLOEXEC flags to open(), I've been too lazy to implement
  the emulation yet.

Signed-off-by: Pierre Habouzit <pierre.habouzit@intersec.com>