bad return code, writing the descriptive message to stderr. Otherwise
the sanitised value is read from stdout. If an input is empty, then
checker may print default value of the option
+* optional executable "apply" file, that is run when "dsc apply" is
+ invoked. It takes a single argument -- option name. If apply is
+ invoked to the list-options directory, then all of them are executed.
+ If apply is invoked for the directory, then all option's in it are
+ applied.
Option having "*" name is treated like a dynamic list.