sh: Add \u/\U support (in $'...') for UTF-8.

sh: Add \u/\U support (in $'...') for UTF-8.

Note that \u/\U are processed using the locale that was active when the
shell started. This is necessary to avoid behaviour that depends on the
parse/execute split (for example when placing braces around an entire
script). Therefore, UTF-8 encoding is implemented manually.

Obtained-from:   FreeBSD SVN rev 221669