Add dmesg functionality.

sys/gdbscripts/dmesg

@@ -0,0 +1,47 @@
+#	$NetBSD: dmesg,v 1.1 2020/04/14 13:58:11 christos Exp $
+
+define dmesg
+	set $mbp = msgbufp
+	set $bufdata = &$mbp->msg_bufc[0]
+	set $print = $mbp->msg_bufs
+	set $newl = 0
+	set $skip = 0
+	set $i = -1 
+	set $p = $bufdata + $mbp->msg_bufx - 1
+
+	while ($i < $mbp->msg_bufs)
+		set $i = $i + 1
+		set $p = $p + 1
+		if ($p == $bufdata + $mbp->msg_bufs)
+			set $p = $bufdata
+		end
+		if ($i < $mbp->msg_bufs - $print)
+			loop_continue
+		end
+		set $c = $p[0]
+		# Skip syslog sequences
+		if ($skip)
+			if ($c == '>')
+				set $newl = 0
+				set $skip = 0
+			end
+			loop_continue
+		end
+		if ($newl && $c == '<')
+			set $skip = 1
+			loop_continue
+		end
+		if ($c == '\0')
+			loop_continue
+		end
+		set $newl = $c == '\n'
+		printf "%c", $c
+	end
+	if (!$newl)
+		printf "\n"
+	end
+end
+
+document dmesg
+print the message buffer
+end