Rework cmsg.rs internals to address portability issues.

Replace our versions of CMSG_SPACE and CMSG_LEN with libc implementations.
Avoid depending on our version of CMSG_ALIGN, since it's non-portable.
Avoid need for align_buf, since we always leave the buffer correctly aligned now.

Tested on x86_64: Linux, FreeBSD, OpenBSD, macOS.

This fixes issue #83.

r? @ChunMinChang please

mozilla / audioipc