Add columns for process autogroup identifier and nice value

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

linux/LinuxProcessList.h

@@ -71,6 +71,7 @@ typedef struct LinuxProcessList_ {
 
    TtyDriver* ttyDrivers;
    bool haveSmapsRollup;
+   bool haveAutogroup;
 
    #ifdef HAVE_DELAYACCT
    struct nl_sock* netlink_socket;