MacOS: "-e filename/." returns true even for non-directories.

This has something to do with resource forks.  So use "-d filename/."
instead, which returns false if filename is not a directory.

t/default.vartest.do

@@ -0,0 +1,2 @@
+: ${PREFIX=not defined}
+echo "$PREFIX"

t/vartest.do

@@ -0,0 +1,3 @@
+PREFIX=/a/b/c/d/e redo vartest2
+read x <vartest2
+[ "$x" = "/a/b/c/d/e" ] || exit 45