Rework in charset code. (Includes renaming.)

Renamed check_charset() to is_native_charset(), and sbr/check_charset.c to
sbr/charset.c. Removed caching in favor for simpler code.
is_native_charset() does not check for prefixes anymore. The code didn't
match the comments anyway and it matched when comparing the first 10 [sic!]
chars of `ISO-8859-1whatever' with `ISO-8859-11', which is not wanted.