Since Linux 2.6.17, epoll is able to report about peer half-closed connection
using special EPOLLRDHUP flag on a read event.

It was introduced in Linux 2.6.39, glibc 2.14 and allows to obtain
file descriptors without actually opening files.  Thus made it possible
to traverse path with openat() syscalls without the need to have read
permissions for path components.  It is effectively emulates O_SEARCH
which is missing on Linux.

O_PATH is used in combination with O_RDONLY.  The last one is ignored
if O_PATH is used, but it allows nginx to not fail when it was built on
modern system (i.e. glibc 2.14+) and run with a kernel older than 2.6.39.
Then O_PATH is unknown to the kernel and ignored, while O_RDONLY is used.

Sadly, fstat() is not working with O_PATH descriptors till Linux 3.6.
As a workaround we fallback to fstatat() with the AT_EMPTY_PATH flag
that was introduced at the same time as O_PATH.

Based on a patch by Piotr Sikora.

Catch up with new Linux version numbering scheme as announced at [1] and
suppress unrecognized versions to actually use default 0.

[1] https://lkml.org/lkml/2011/5/29/204