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.

src/basic/process-util.c

@@ -1337,6 +1337,13 @@ int safe_fork_full(
                         log_full_errno(prio, r, "Failed to connect stdin/stdout to /dev/null: %m");
                         _exit(EXIT_FAILURE);
                 }
+
+        } else if (flags & FORK_STDOUT_TO_STDERR) {
+
+                if (dup2(STDERR_FILENO, STDOUT_FILENO) < 0) {
+                        log_full_errno(prio, r, "Failed to connect stdout to stderr: %m");
+                        _exit(EXIT_FAILURE);
+                }
         }
 
         if (flags & FORK_RLIMIT_NOFILE_SAFE) {