fix "*** buffer overflow detected ***: terminated Aborted" by adding posix poll() io implementation in addition to select()

[pmurzakov]

select() API has a major drawback which doesn't allow to work with fd numbers bigger than 1024. It's a really small number nowadays as the fd table is shared between open files, sockets, connections and so on. While it might be ok for small projects it is critical for high-load projects with a lot of connections open simultaneously or for projects which work with a lot of files dealing with memcache at the same time.

Here's a quote from from first lines of documentation for select():

WARNING: select() can monitor only file descriptors numbers that
are less than FD_SETSIZE (1024)—an unreasonably low limit for
many modern applications—and this limitation will not change.
All modern applications should instead use poll(2) or epoll(7),
which do not suffer this limitation.

https://man7.org/linux/man-pages/man2/select.2.html

Here's a reproduce case:

<?php
for ($i = 0; $i < 1024; $i++) {
        $f[] = fopen("/tmp/testfd_$i", 'a'); // the same effect with files, sockets, open connections, etc
}

$m = memcache_connect('localhost');
if ($m !== false) {
	$a = memcache_get($m, 'key');
}

here's result:

php -f 1.php
*** buffer overflow detected ***: terminated
Aborted (core dumped)

The PR adds option memcache.select_api which is one of poll or select (default). On Win32 only select is supported for now. While it is technically possible to implement it for win32 as it should be available starting from Windows Vista I left it for further scope as I don't have relevant expertise working with Windows APIs.

[pmurzakov]

Sorry, I closed it by accident