bug#13041: 24.2; diacritic-fold-search

[Juri Linkov]

> `ignore-diacritics' is misleading.  The variable would have to be called
> `observe-decompositions' or something the like.

Since the existing variable that corresponds to the
Unicode file CaseFolding.txt is `case-fold-search',
its counterpart variable that corresponds to the Unicode file
Decomposition.txt could be called `decomposition-search'.

Also like the existing `sort-fold-case', its counterpart could be called
`sort-decomposition'.