Use a bit safer name for data source

... against conflict when -v is used and the name is exactly this.

src/include/pretty.sh

     test "$frontend" == "main" && frontend=$(basename $0)
     test "$caller" == "main" && caller=$(basename $0)
     test "$frontend" == "usage" && frontend=usage_is
-    local src       # which source, or "producer" (__echo_*()) to use
+    local provider       # which provider (__echo_*()) to use
     while true; do case $1 in
-        -c|--cmd)   src=cmd;   shift; break ;;
-        -f|--files) src=files; shift; break ;;
-        -l|--lines) src=lines; shift; break ;;
-        -t|--trace) src=trace; shift; break ;;
-        -u|--usage) src=usage; shift; break ;;
-        -v|--vars)  src=vars;  shift; break ;;
-        *)          src=args;         break ;;
+        -c|--cmd)   provider=cmd;   shift; break ;;
+        -f|--files) provider=files; shift; break ;;
+        -l|--lines) provider=lines; shift; break ;;
+        -t|--trace) provider=trace; shift; break ;;
+        -u|--usage) provider=usage; shift; break ;;
+        -v|--vars)  provider=vars;  shift; break ;;
+        *)          provider=args;         break ;;
     esac done
     case $frontend in
         debug)    ;&
         think)    ;&
         usage_is) ;&
         warn)
-            __echo_$src "$@" | _pretty_$frontend >&2
+            __echo_$provider "$@" | _pretty_$frontend >&2
             ;;
         *)
             echo "do not call __echo* directly: $frontend" >&2