locale: Fix signed char bug in lr_getc

The array lr->buf contains characters, which can be signed.  A 0xff
byte in the input could be incorrectly reported as EOF.  More
importantly, get_string in linereader.c converts a signed input byte
to a Unicode code point using ADDWC ((uint32_t) ch), under the
assumption that this decodes the ISO-8859-1 input encoding.  If char
is signed, this does not give the correct result.  This means that
ISO-8859-1 input files for localedef are not actually supported,
contrary to the comment in get_string.  This is a happy accident because
we can therefore change the file encoding to UTF-8 without impacting
backwards compatibility.

While at it, remove the \32 check for MS-DOS end-of-file character (^Z).

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>