process-util: add new safe_fork() flag for connecting stdout to stderr

process-util: add new safe_fork() flag for connecting stdout to stderr

This adds a new safe_fork() flag. If set the child process' fd 1 becomes
fd 2 of the caller. This is useful for invoking tools (such as various
mkfs/fsck implementations) that output status messages to stdout, but
which we invoke and don't want to pollute stdout with their output.