score-level headers made global

[Nicolas Sceaux]

Using LilyPond from CVS.
In the following example, the two pieces get wrong titling:

piece 1          opus 2
=======================

piece 1          opus 2
=======================

instead of:

piece 1          opus 1
=======================

                 opus 2
=======================

If the first (book level) \header block is removed, then the two score
titles are OK.

\version "2.7.5"
\header {
    tagline = ""
    copyright = ""
}

\score {
  { c'1 }
  \header {
    piece = "piece 1"
    opus = "opus 1"
  }
}

\score {
  { d'1 }
  \header {
    opus = "opus 2"
  }
}