Revert "Handling collision between standard i/o file descriptors and newly created ones"

g_unix_open_pipe tries to avoid the standard io fd range when getting pipe fds. This turns out to be a bad idea because certain buggy programs rely on it using that range.

This reverts commit d9ba6150 and adds a test to ensure we don't inadvertently do it again later.

Closes: #2795 (closed)