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

pcp/Platform.h

@@ -210,6 +210,9 @@ typedef enum Metric_ {
    PCP_PROC_EXE,                /* proc.psinfo.exe */
    PCP_PROC_CWD,                /* proc.psinfo.cwd */
 
+   PCP_PROC_AUTOGROUP_ID,       /* proc.autogroup.id */
+   PCP_PROC_AUTOGROUP_NICE,     /* proc.autogroup.nice */
+
    PCP_PROC_ID_UID,             /* proc.id.uid */
    PCP_PROC_ID_USER,            /* proc.id.uid_nm */