The new config value is duplicated, and the copy is stored to the
configuration. After the configuration R/W lock is released, we have no
warranty that another thread does not change the same configuration
item, and free our own copy. Admittedly, this is very unlikely.

Instead, we can simply pass the original string from the caller to the
callback - that one must remain valid through the config_PutPsz()
function call by definition.