Add CTLFLAG_STATS to sysctl flags

In preparation for sysctl(8) growing the ability to only print
out boot/run-time tunables we need a way to differentiate between
RW sysctl nodes that tune a particular thing, or simply export
a stat that we want to allow the sysadmin to reset to 0 (or some
other value).

To do so, we add the CTLFLAG_STATS which should be OR'd into the
CTLFLAGs when exporting a "writable/resettable" statistic node via
sysctl.

sys/sys/sysctl.h

@@ -90,6 +90,7 @@ struct ctlname {
 #define	CTLFLAG_DYING	0x00010000	/* oid is being removed */
 #define CTLFLAG_CAPRD	0x00008000	/* Can be read in capability mode */
 #define CTLFLAG_CAPWR	0x00004000	/* Can be written in capability mode */
+#define CTLFLAG_STATS	0x00002000	/* Statistics, not a tuneable */
 #define CTLFLAG_CAPRW	(CTLFLAG_CAPRD|CTLFLAG_CAPWR)
 
 /*