Find coding system corresponding to CHARSET.
CHARSET is any sort of non-Emacs charset name, such as might be used
in a locale codeset, or elsewhere. It is matched to a coding system
first by case-insensitive lookup in `locale-charset-alist'. Then
matches are looked for in the coding system list, treating case and
the characters `-' and `_' as insignificant. The coding system base
is returned. Thus, for instance, if charset "ISO8859-2",
`iso-latin-2' is returned.