mkofs improvements

- Always use $PWD/o. if it doesn't exist, say how to create it.

- return 2 on unsupported platforms.

tool/zsh/mkofs

@@ -1,17 +1,16 @@
 [[ -f /proc/mounts ]] || {
   echo "mkofs: only supported on linux" >&2
-  return 1
+  return 2
 }
 local o
-if [[ -d "$PWD/o" ]]; then
-  o=$PWD/o
-elif [[ -n "$COSMO" && -d "$COSMO/o" ]]; then
-  o=$COSMO/o
+if [[ -d o ]]; then
+  o=$(pwd -P)/o
 else
   echo "mkofs: o/ not found" >&2
+  echo "suggested preparation:" >&2
+  echo "    mkdir -m 555 o" >&2
   return 1
 fi
-o=$(realpath "$o")
 cut -d' ' -f2 /proc/mounts | while read -r line; do
   if [[ "$o" = "$line" ]]; then
     echo "mkofs: $o already mounted" >&2