Child.start_suspended ported to posix

[GiuseppeCesarano]

This patch ports Child.start_suspended to Linux.

Note: The doc-comment says that start_suspended is implemented for Darwin but giving a quick look to the file I can't find where this is achieved.

[GiuseppeCesarano]

Following up the comment in the review, I can add std.posix.getpid to the std lib if you confirm that calling system.getpid in std.posix is a sufficient for other targets which are not Linux.

[alexrp]

See #24027. Perhaps you can revive that one since the author hasn't responded to my review comments yet.

[GiuseppeCesarano]

I've used wait4 for riscv since there ins't the waitpid const in the syscalls consts.

But please note that wait4 is not a standard posix function but a BSD/Linux extension so I don't know if that's ok without restricting the the OS

[alexrp]

Looks good.

For the sake of clean history, can you squash this into two commits:

• The addition of getpid and getppid to std.posix
• The implementation of POSIX start_suspended support in std.process.Child

[alexrp]

Thanks!