(stable) Call setsid() unconditionally after daemonizing

Ref T12355. This was alright locally, but in production the system is not
thrilled about posix_isatty() from the daemonized process:

ERROR 2: posix_isatty(): expects argument 1 to be a valid stream resource

After daemonizing we never expect STDOUT to be a TTY, so skip the check.

Auditors: chad