Merge branch 'jc/same-encoding' into maint

Merge branch 'jc/same-encoding' into maint

Various codepaths checked if two encoding names are the same using
ad-hoc code and some of them ended up asking iconv() to convert
between "utf8" and "UTF-8".  The former is not a valid way to spell
the encoding name, but often people use it by mistake, and we
equated them in some but not all codepaths. Introduce a new helper
function to make these codepaths consistent.

* jc/same-encoding:
  reencode_string(): introduce and use same_encoding()