Adds a new property in multifdsink, resend-streamheader.

If this property is false, the multifdsink will not send the streamheader if
there's already one set for a particular client.

There are some formats in which every stream needs to start with a certain
blob, but you can't inject this blob at leisure. If the producer wants to
change the blob in question and sets in as the streamheader on the outgoing
buffers' caps, new clients of multifdsink will get the new streamheader, but
old clients will break, because they'll see the blob in the middle of the
stream.

The property is true by default, so existing code will not see any difference.

Fixes #578118.