Auto merge of rust-lang#3720 - safinaskar:read, r=RalfJung

Fix libc::read shim: make it write to a buffer correct amount of bytes. Add tests for new behavior

libc::read shim had a bug: if underlying real call libc::read(fd, buf, N) returns M, then
libc::read shim writes N bytes to buf instead of M. Remaining N - M bytes are filled with zeros.
This commit fixes this bug and adds tests for new behavior

Author: bors