Improve errno detection for failed connections without ext-sockets

[clue]

This changeset improves errno detection for failed connections without ext-sockets. Reporting the errstr already works on most platforms even when ext-sockets is not installed, consistently reporting the correct errno previously required ext-sockets. With these changes applied, we now also take the errno constants from ext-pcntl and ext-posix into account. This is particularly useful for lightweight container images (Docker) that by default come with ext-sockets and ext-pcntl disabled but ext-posix enabled. All extensions remain entirely optional.

Link dump:

• https://man7.org/linux/man-pages/man3/errno.3.html
• https://man7.org/linux/man-pages/man3/strerror.3.html
• https://github.com/torvalds/linux/blob/master/include/uapi/asm-generic/errno.h
• https://learn.microsoft.com/en-us/cpp/c-runtime-library/errno-constants?view=msvc-170

Builds on top of #271, #267, #266 and others