statvfs(3) bindings

libc-test/build.rs

src/unix/bsd/apple/mod.rs

src/unix/bsd/freebsdlike/mod.rs

src/unix/bsd/mod.rs

src/unix/bsd/openbsdlike/mod.rs

src/unix/mod.rs

src/unix/notbsd/linux/mod.rs

src/unix/notbsd/linux/notmusl.rs