Ban @ from xcase__enum()

Turns out that case ids may end up being re-used in URI contexts, and
'@' has special meaning in some parts of URL (HTTP, at least), which
could cause confusion or make translations to HTTP URLs troublesome.

Since it looks like it's not used anywhere yet, it's safer to drop it.

src/xcase.sh.skel

     #
     # Make sure stuff from xcase__enum() has no banned chars
     #
-    local allowed='[:alnum:]._,+%@=-'   # allowed chars in case id
+    local allowed='[:alnum:]._,+%=-'    # allowed chars in case id
     local es=                           # exit status of this function
     if grep "[^$allowed]" "$__xcase__tmp/enum";
     then