Constants for the linux nfqueue netlink protocol (userspace firewall).

Continuation of #1562 and #1571.

fix #1633

Ref: https://android.googlesource.com/platform/bionic/+/refs/tags/ndk-r20/libc/include/sys/ucontext.h

Ref: https://android.googlesource.com/platform/bionic/+/refs/tags/ndk-r20/libc/include/sys/ucontext.h

Ref: https://android.googlesource.com/platform/bionic/+/refs/tags/ndk-r20/libc/include/sys/ucontext.h

They are missing in the CI’s linux-api-headers, and not widely available
yet either.

fix #1632

memmem is a non-standard extension, first added in GNU libc and later
ported to other systems. Support for it is non-uniform, thus it was only
added to platforms that seem to support it.

This triggers a warning on a recent nightly, which in turn breaks CI due to
`#![deny(warnings)]` in libc-test/build.rs

This commit performs a number of updates for libc with the `wasm32-wasi`
target:

* Updates the `wasi-libc` repository commit used (previously known as
  `wasi-sysroot`)
* Updates the container to Ubuntu 19.10 which has Clang 9 packaged which
  is all we need.
* Avoids building `wasmtime` and instead downloads a precompiled binary.
* Updates bindings in `src/wasi.rs` to match the current upstream state.

sock_extended_err is a struct returned as a control message when the sockopt
IP_RECVERR is set, when recvmsg has the MSG_ERRQUEUE flag set.  IP_RECVERR and
MSG_ERRQUEUE are constants both already defined here.