Make sure a spawned process has all standard fds connected.

* src/posix-io.c (_gpgme_io_spawn): dup /dev/null also to unsued
stdout.

Better be sure that stdout of a spawned process is connected to stdout
so that the process does not run into a write error if it writes to
stdout. AFAICS we always use a connected stdout; thus this is only
for correctness.