Split out SIGSYS handling and redirect accept to accept4

[marksteward]

This fixes SIGSYS on Oreo for processes that call accept. I can't see a recent change to bionic, but accept4 definitely seems to be the supported call.

I'm not massively familiar with this codebase, and I've basically pulled out the code from 454b0b1 and chain.c. It took a while to work out something that does the right thing, so it's likely I've misunderstood how it should work or overlooked a corner case.

Tested on aarch64, Android 8.0.0, kernel 3.18.66-perf targetting nc -l -p 8080 and chromium-browser --headless on Ubuntu.

[marksteward]

On a side note, I've been building and testing this on termux, and it seems that the clang toolchain includes some changes that break the build process. Should I create a PR to fix that?

[michalbednarski]

Thanks, I'll test this myself and merge then

If build doesn't work for you and/or you've made changes that allow it to be built on something else than what I'm using/be built on device, that pull request is are also welcome.

I'm usually building this through termux-packages and then install .deb on device. For development (when not using version pushed to git repo) appending following to packages/proot/build.sh can be used to build local version:

termux_step_extract_package () {
	cp -r /home/.../proot $TERMUX_PKG_SRCDIR
}