child_process: fix handle delivery

Commit 9352c19 ("child_process: don't emit same handle twice") trades
one bug for another.

Before said commit, a handle was sometimes delivered with messages it
didn't belong to.

The bug fix introduced another bug that needs some explaining. On UNIX
systems, handles are basically file descriptors that are passed around
with the sendmsg() and recvmsg() system calls, using auxiliary data
(SCM_RIGHTS) as the transport.

node.js and libuv depend on the fact that none of the supported systems
ever emit more than one SCM_RIGHTS message from a recvmsg() syscall.
That assumption is something we should probably address someday for the
sake of portability but that's a separate discussion.

So, SCM_RIGHTS messages are never coalesced. SCM_RIGHTS and normal
messages however _are_ coalesced. That is, recvmsg() might return this:

  recvmsg();  // { "message-with-fd", "message", "message" }

The operating system implicitly breaks pending messages along
SCM_RIGHTS boundaries. Most Unices break before such messages but Linux
also breaks _after_ them.  When the sender looks like this:

  sendmsg("message");
  sendmsg("message-with-fd");
  sendmsg("message");

Then on most Unices the receiver sees messages arriving like this:

  recvmsg();  // { "message" }
  recvmsg();  // { "message-with-fd", "message" }

The bug fix in commit 9352c19 assumes this behavior. On Linux however,
those messages can also come in like this:

  recvmsg();  // { "message", "message-with-fd" }
  recvmsg();  // { "message" }

In other words, it's incorrect to assume that the file descriptor is
always attached to the first message. This commit makes node wise up.

Fixes #5330.

Suggested reviewer: @isaacs