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/ProcessField.h

@@ -44,8 +44,8 @@ in the source distribution for its full text.
    M_PSSWP = 121,                \
    CTXT = 122,                   \
    SECATTR = 123,                \
-                                 \
-   DUMMY_BUMP_FIELD = CWD,       \
+   AUTOGROUP_ID = 127,           \
+   AUTOGROUP_NICE = 128,         \
    // End of list