Dpkg::IPC: Defer filehandle closures in spawn() to avoid double-close

When the caller passes the same filehandle for STDIN, STDOUT or STDERR,
the code will try to close the same filehandle more than once, producing
an error. Defer the closures to the end using the same close_in_child
array already used for other filehandles.

Closes: #839905, #840293