Set SA_ONSTACK in zend_sigaction External

Hi Kevin,

Master should be targeted, no RFC is required, and the patch looks fine to
me.

Cheers
Joe

Hi, internals!

It's been a while.

I'm currently working on a new SAPI for web servers written in Go.
Many virtual machines, including Go (
https://pkg.go.dev/os/signal#hdr-Go_programs_that_use_cgo_or_SWIG), depend
on signals using SA_ONSTACK (
https://man7.org/linux/man-pages/man2/sigaltstack.2.html). This flag
allows
a thread to define a new alternate signal stack. Many argue that SA_ONSTACK
should be a default, but it's not the case (yet).

Python merged a patch setting SA_ONSTACK in 2021 (Python 3.10+) for the
same reasons (https://bugs.python.org/issue43390 /

https://github.com/python/cpython/commit/02ac6f41e5569ec28d625bb005155903f64cc9ee
),
with no issues.

I opened a Pull Request to set this flag by default and tested it
successfully with my Go SAPI: https://github.com/php/php-src/pull/9597

As this is technically at the limit between a new feature and a bug fix
(having the ability to call Go/C++ VM code from PHP and embed PHP in such
programs), should I open an RFC? Also, if merging my patch is considered,
which branch should I target?

Cheers,

Kévin Dunglas