another NULL/reference hole in the API

[Paul Davis <paul linuxaudiosystems com>]

I've just discovered that all the spawn functions in glibmm make it
impossible to pass NULL to the underlying C layer to indicate "use same
working directory as parent". This is another instance of a general
pattern/hole that has been mentioned recently. I don't recall what
Murray's position on it is. Do we have a general policy?

--p