basic/util: move execute_directory() to separate file

It's a fairly specialized function. Let's make new files for it and the tests.

.gitignore

@@ -194,6 +194,7 @@
 /test-env-util
 /test-escape
 /test-event
+/test-exec-util
 /test-execute
 /test-extract-word
 /test-fd-util