unicode: check $LC_ALL to detect Unicode mode, not only $LANG

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
foo*sum: report I/O errors, don't merely exit with 1.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
tail: track file size only in -f mode

This eliminates extra fstat and lseek calls on every read

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>