Do not exclude anything just if debug is on and caller fits

The exclusion criterion would be true for amy frontend just if debug
was on and the caller fit SHELLFU_DEBUG_EXCLUDE.  That is, even warnings
and thoughts from the caller would be muted!

In fact, it's frontend that matters; only debug frontend should be
considered for exclusion.  OTOH, we don't need to check SHELLFU_DEBUG as
debug frontends already give up ASAP if that is not set.

We also need to move the frontend rewrite sooner for this to work.

src/include/pretty.sh

     local provider  # which provider (_pretty__echo_*()) to use
     frontend="$(_pretty__get_frontend)" || exit_usage
     caller="$(_pretty__get_caller)"
-    $SHELLFU_DEBUG && _pretty__is_excluded "$caller" && return 0
+    test "$frontend" == "debug_pipe()"  && frontend=debug
+    test "$frontend" = debug && _pretty__is_excluded "$caller" && return 0
     test "$caller"   == "main()"        && caller="$(basename "$0")"
     test "$caller"   == "usage()"       && frontend=mkusage
-    test "$frontend" == "debug_pipe()"  && frontend=debug
     while true; do case $1 in
         -c|--cmd)   provider=cmd;   shift; break ;;
         -f|--files) provider=files; shift; break ;;