Adds AGRP (autogroup) and ANI (autogroup nice) columns that
report the information from /proc/PID/autogroup, as well as
handlers for '{' and '}' to change the autogroup nice value.
This is guarded by /proc/sys/kernel/sched_autogroup_enabled
such that sampling and/or changing values wont be attempted
unless the kernel feature is enabled.
Fixes: #720
1bd95983b2