pass_fd:  Rename cmsg_alloc to cmsg_fd_alloc

It's better documentation and we don't want to risk a collision with any
future cmsg types in unet.
[FIX] pass_fd:  Avoid CMSG_LEN() in array bounds

This works fine in g++ 4.0, clang, and Metrowerks, but g++ 4.2 hates it.