start-stop-daemon, supervise-daemon: set autogroup nicelevel

As described in "Why nice levels are a placebo and have been for a very
long time, and no one seems to have noticed"[1], the Linux kernel in its
default configuration on many Linux distributions autogroups tasks by
session ID and "fairly" allocates CPU time among such autogroups. The
nice levels of tasks within each autogroup are only relative to
other tasks within the same autogroup. Effectively, this means that the
traditional nice level is rendered moot for tools like start-stop-daemon
and supervise-daemon, which start each daemon in its own session and
thus in its own autogroup. Linux does provide a means to change the
niceness of autogroups relative to each other, so let's have start-stop-
daemon and supervise-daemon make use of this feature where available so
that -N,--nicelevel/SSD_NICELEVEL will actually do what the user
intends. On systems where autogroups are not supported or are disabled,
this commit introduces no change in behavior.

Note that the setsid() call in the child process of start-stop-daemon is
moved to much earlier. This is necessary so that the new process will be
assigned to a new autogroup before the autogroup nicelevel is set. To
avoid inadvertently acquiring /dev/tty as the controlling terminal of
the new session after setsid() has given up the controlling terminal
inherited from the parent process, tty_fd is opened before the call to
setsid().

[1] https://www.reddit.com/r/linux/comments/d7hx2c/why_nice_levels_are_a_placebo_and_have_been_for_a/
This fixes #542.

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