systemd: ignore lack of tty when checking whether colors should be enabled

systemd: ignore lack of tty when checking whether colors should be enabled

When started by the kernel, we are connected to the console, and we'll set TERM
properly to some value in fixup_environment(). We'll then enable or disable
colors based on the value of $SYSTEMD_COLORS and $TERM.

When reexecuting, TERM should be already set, so we can use this value.
Effectively, behaviour is the same as before affd7ed1a was reverted, but instead
of reopening the console before configuring color output, we just ignore what
stdout is connected to and decide based on the variables only.