Change NamedTempScript to NamedTempFile; allow streaming to it.

The only thing about NamedTempScript that was specific to script files was the
hardcoded ".py" extension. Renaming it to NamedTempFile with an explicit
extension argument addresses that.
Allow constructing NamedTempFile with either a std::string, as before, or an
expression of the form (lambda::_1 << some << stuff). If Linden's Boost
package included the Boost.Iostreams lib, we could even stream such an
expression directly to an ostream constructed around the fd. But oh well.