i18n: Introduce C_ENCODING as replacement for LEGACY_NATIVE.

The LEGACY_NATIVE name seems a bit awkward for something that is just the
name of the encoding for strings in C source code, that is, the C locale,
so this commit renames it to C_ENCODING and moves it to i18n.h with the
rest of the encoding-related functions.

In addition, PSPP assumes in various places that the local system has
ASCII-based locales.  I don't think there's much point in pretending to
support EBCDIC, so this commit removes that little bit of support.