os: use the correct constant for P_PID on NetBSD

Dragonfly and FreeBSD both used numerical values for these constants
chosen to be the same as on Solaris. For some reason, NetBSD did not,
and happens to interpret value 0 as P_ALL instead of P_PID
(see https://github.com/NetBSD/src/blob/3323ceb7822f98b3d2693aa26fd55c4ded6d8ba4/sys/sys/idtype.h#L43-L44).

Using the correct value for P_PID should cause wait6 to wait for the
correct process, which may help to avoid the deadlocks reported in

For #50138.
Updates #13987.

Change-Id: I0eacd1faee4a430d431fe48f9ccf837f49c42f39
Reviewed-on: https://go-review.googlesource.com/c/go/+/442478
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/os/wait_wait6.go

@@ -11,7 +11,10 @@ import (
 	"syscall"
 )
 
-const _P_PID = 0
+const (
+	_P_PID        = 0 // everywhere except for NetBSD?
+	_P_PID_NETBSD = 1 // on NetBSD, 0 is P_ALL
+)
 
 // blockUntilWaitable attempts to block until a call to p.Wait will
 // succeed immediately, and reports whether it has done so.
@@ -26,6 +29,8 @@ func (p *Process) blockUntilWaitable() (bool, error) {
 			_, _, errno = syscall.Syscall9(syscall.SYS_WAIT6, _P_PID, uintptr(p.Pid), 0, 0, syscall.WEXITED|syscall.WNOWAIT, 0, 0, 0, 0)
 		} else if runtime.GOOS == "freebsd" && runtime.GOARCH == "arm" {
 			_, _, errno = syscall.Syscall9(syscall.SYS_WAIT6, _P_PID, 0, uintptr(p.Pid), 0, 0, syscall.WEXITED|syscall.WNOWAIT, 0, 0, 0)
+		} else if runtime.GOOS == "netbsd" {
+			_, _, errno = syscall.Syscall6(syscall.SYS_WAIT6, _P_PID_NETBSD, uintptr(p.Pid), 0, syscall.WEXITED|syscall.WNOWAIT, 0, 0)
 		} else {
 			_, _, errno = syscall.Syscall6(syscall.SYS_WAIT6, _P_PID, uintptr(p.Pid), 0, syscall.WEXITED|syscall.WNOWAIT, 0, 0)
 		}