child_process: fix handle delivery

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.

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.

This is a back-port of commit 21bd456 from the v0.10 branch. The test
has been dropped as it's not compatible with the v0.8 process model.

Fixes #5330.

Conflicts:
	lib/child_process.js