runtime: make NumGoroutine and Stack agree not to include system goroutines

Before, NumGoroutine counted system goroutines and Stack (usually) didn't show them,
which was inconsistent and confusing.

To resolve which way they should be consistent, it seems like

	package main
	import "runtime"
	func main() { println(runtime.NumGoroutine()) }

should print 1 regardless of internal runtime details. Make it so.

Fixes #11706.

Change-Id: I6bfe26a901de517728192cfb26a5568c4ef4fe47
Reviewed-on: https://go-review.googlesource.com/18343
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/proc.go

@@ -2162,6 +2162,9 @@ func goexit0(gp *g) {
 	_g_ := getg()
 
 	casgstatus(gp, _Grunning, _Gdead)
+	if isSystemGoroutine(gp) {
+		atomic.Xadd(&sched.ngsys, -1)
+	}
 	gp.m = nil
 	gp.lockedm = nil
 	_g_.m.lockedg = nil
@@ -2693,6 +2696,9 @@ func newproc1(fn *funcval, argp *uint8, narg int32, nret int32, callerpc uintptr
 	gostartcallfn(&newg.sched, fn)
 	newg.gopc = callerpc
 	newg.startpc = fn.fn
+	if isSystemGoroutine(newg) {
+		atomic.Xadd(&sched.ngsys, +1)
+	}
 	casgstatus(newg, _Gdead, _Grunnable)
 
 	if _p_.goidcache == _p_.goidcacheend {
@@ -2885,7 +2891,7 @@ func badunlockosthread() {
 }
 
 func gcount() int32 {
-	n := int32(allglen) - sched.ngfree
+	n := int32(allglen) - sched.ngfree - int32(atomic.Load(&sched.ngsys))
 	for i := 0; ; i++ {
 		_p_ := allp[i]
 		if _p_ == nil {