Custom background select needs a contextualized label

[xibe]

In /themes.php?page=custom-background, the fixed/scroll selector uses the 'Attachment' label.

Problem is, that label name is also used for another type of attachment: files.

Ergo, translators might find themselves with an incorrect wording. In effect, the French translation displays "Fichier attaché" (attached file) instead of a more appropriate term.

I suggest turning the string into a contextualized one.

Another solution would be to change the wording altogether, for instance: "Behaviour when scrolling", with "Scroll along" and "Keep in place" as its options.

Hence, I submit both (hopefully correct) patches -- built against trunk for now, but the issue obviously exists in 3.5.

Patch for context, against trunk.

Patch for name change, against trunk.