Move the static and const declarations to the top of the file

Rename platform/linux to platform/unix in preparation to adding support
for FreeBSD.

Instead of directly using ptr::offset and the equivalent of CMSG_DATA in
glibc on Linux, create a CMSG_DATA function that should work on Linux
and FreeBSD. This should not cause any side effects, but should make it
easier if we should introduce support for another OS that may have a
different implementation of CMSG_DATA.

Add the new_sockaddr_un function that builds a sockaddr_un struct in a
platform agnostic way.

Add OS specific types that address alignment problems and their
respective libc definitions.