Update MUSL Linux kernel headers and re-enable Linux tests

.travis.yml

ci/docker/aarch64-unknown-linux-musl/Dockerfile

ci/docker/arm-unknown-linux-musleabihf/Dockerfile

ci/docker/i686-unknown-linux-musl/Dockerfile

ci/docker/mips-unknown-linux-gnu/Dockerfile

ci/docker/mips-unknown-linux-musl/Dockerfile

ci/docker/mips64-unknown-linux-gnuabi64/Dockerfile

ci/docker/mips64el-unknown-linux-gnuabi64/Dockerfile

ci/docker/x86_64-unknown-linux-gnu/Dockerfile

ci/docker/x86_64-unknown-linux-musl/Dockerfile

ci/install-musl.sh

libc-test/Cargo.toml

libc-test/build.rs

libc-test/test/linux_elf.rs

libc-test/test/linux_ipv6.rs

libc-test/test/linux_strerror_r.rs

libc-test/test/linux_termios.rs

src/unix/bsd/mod.rs

src/unix/haiku/mod.rs

src/unix/hermit/mod.rs