Fix poll() if POLLIN == ROLLRDNORM|POLLRDBAND

Fix poll() if POLLIN == ROLLRDNORM|POLLRDBAND

It seems like POLLIN is specified as equivalent to POLLRDNORM | POLLRDBAND. Some
systems (e.g. QNX and HP-UX) take this literaly and have POLLIN defined as the
above bit combination. Other systems (e.g. Linux) have POLLIN as just a single
bit.

This means that if no out-of-band data is available (which should never be the
case), the result of poll() will not fulfil (fd.revents & POLLIN) == POLLIN on
QNX, because the POLLRDBAND bit is not set.

In other words, even though poll() signaled that the fd is readable, xcb would
not read from the file descriptor.

Fix this by checking if any bits from POLLIN are set in the result of poll(),
instead of all of them.

(This change was independently done by seanb@qnx.com as well)

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=38001
Acked-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Uli Schlachter <psychon@znc.in>