locales: Enforce US-ASCII encoding (limited to 7-bit)

The US-ASCII format was getting treated identically to POSIX. It is
supposed to throw an ILSEQ errno if a value of 0x80 or greater is
encountered, so let's bring back the "ASCII" handling.

While here, change nl_codeset to return US-ASCII only when the encoding
really is "US-ASCII". Before "C" and "POSIX" encoding returned this
string, so now they return "POSIX".

Discussed with: ache
Submitted by: marino
Obtained from: DragonflyBSD