runtime: readjust signal code for dragonfly-2.6 and above

Also adds missing nosplit to unminit.

Fixes #13964.

Change-Id: I07d93a8c872a255a89f91f808b66c889f0a6a69c
Reviewed-on: https://go-review.googlesource.com/18658Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os1_dragonfly.go

@@ -168,6 +168,7 @@ func minit() {
 }
 
 // Called from dropm to undo the effect of an minit.
+//go:nosplit
 func unminit() {
 	if getg().m.newSigstack {
 		signalstack(nil)

src/runtime/os2_dragonfly.go

@@ -6,7 +6,7 @@ package runtime
 
 const (
 	_NSIG        = 33
-	_SI_USER     = 0x10001
+	_SI_USER     = 0
 	_SS_DISABLE  = 4
 	_RLIMIT_AS   = 10
 	_SIG_BLOCK   = 1