main: use strdup instead of free_and_strdup to initialize default unit (#4335)

main: use strdup instead of free_and_strdup to initialize default unit (#4335)

Previously we've used free_and_strdup() to fill arg_default_unit with unit
name, If we didn't pass default unit name through a kernel command line or
command line arguments. But we can use just strdup() instead of
free_and_strdup() for this, because we will start fill arg_default_unit
only if it wasn't set before.