Change default socket backlog size to 511

[clue]

The backlog parameter used to default to just 32 as of PHP 5.3.3 and
used to be hard coded to 4 in earlier PHP versions.

The new default of 511 was evaluated to be a reasonable value for
event-driven applications that can cope with many concurrent connections
on a variety of platforms. In particular, this solves common ECONNRESET
(connection reset by peer) errors when running benchmarks with higher
concurrency. For more specific requirements, the backlog parameter can
be given explicitly to override this default.

See php/php-src@5b277c4
Refs reactphp/http#354 and others

[kelunik]

@clue Care to shed some light why you chose 511 instead of 512?

[clue]

@acasademont 511 is not only used in Node.js and PHP-FPM, but also in Nginx, Apache, Redis, etc... I'd say 511 was determined to be a safe limit on most OSes (except FreeBSD where it's -1). The backlog limit default in PHP was committed 5 years ago by an user, I don't think there was much discussion about it (the limit default itself) before or after adding a backlog stream context option?

Originally posted by @CharlotteDunois in reactphp/http#354 (comment)

[kelunik]

Thanks!