Avoid infinite loop in NetlinkListener.

Currently, NetlinkListener only reads from its socket if it gets
POLLIN. This means that if the kernel returns POLLERR without
POLLIN, it will get into an infinite loop.

Fix this by responding to POLLERR by issuing a recvfrom, which
clears the socket error, and continuing. The only error we expect
to see here is ENOBUFS, and there's nothing we can do about that.
There is no functional change because before we'd just call
forEachNetlinkMessage on an empty Slice, which does nothing.

Also increase the socket buffer to 1MB to reduce the chance of
getting ENOBUFS and thus leaking some mCookieTagMap entries. This
buffer size is equivalent to half the buffer size of a single TCP
socket on LTE.

While we're at it, don't pass POLLRDHUP, POLLERR, and POLLHUP to
poll, since these are ignored in events and are only meaningful
in revents.

Bug: 111479770
Test: netd_{unit,integration}_test pass
Test: builds, boots, cell and wifi work
Test: stress test does not cause infinite loop in netd
Change-Id: I847aeb9a53095c1dfdeddadcd20c0e750b6513ff