PORTAGE_NICENESS: Consider autogroup scheduling

With Linux's autogroup scheduling feature (CONFIG_SCHED_AUTOGROUP)
setting a nice value on a per-process base has only an effect for
scheduling decisions relative to the other threads in the same
session (typically: the same terminal window). See the section "The
nice value and group scheduling" in the sched(7) man page.

Basically this means that portage "just" setting the nice value, has
no effect in presence of autogroup scheduling being active (which is
probably true for most (desktop) user systems).

This commit changes emerge to set the autogroup's nice value, instead
of the processes' nice value, in case autogroups are present (detected
by the existence of /proc/self/autogroup). The tricky part about
autogroup nice values is that we want restore the orignal nice value
once we are finished. As otherwise, the session, e.g. your terminal,
would continue using this value, and so would subsequently executed
processes. For that we use Python's atexit functinaly, to register a
function that will restore the orignal nice value of the autogroup.

Users may have set PORTAGE_NICENESS to a value outside of the range
of valid nice values [-20, 19]. Calling os.nice() with such a value
will simply cap the process's nice value, but writing this invalid
value to the autogoup pseudo-file will fail with "Invalid argument".
Since os.nice() returns the current nice value, we simply use the
returned value to set the autogroup nice value.

Portage would previously always change the nice value to zero, even if
the user did not explicitly request so. Now we do not change the nice
value unless requested.

Closes: https://github.com/gentoo/portage/pull/727
Bug: https://bugs.gentoo.org/777492
Signed-off-by: Florian Schmaus <flo@geekplace.eu>
Signed-off-by: Zac Medico <zmedico@gentoo.org>

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