add stallion's rdb helper mentioned in rdb(8)

Reference: /n/atom/patch/applied/rdbhelper
Date: Tue Mar 11 15:36:21 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /rc/bin/rdb	Thu Jan  1 00:00:00 1970
+++ /rc/bin/rdb	Tue Mar 11 15:35:41 2014
@@ -0,0 +1,51 @@
+#!/bin/rc
+# rdb - remote kernel debugging
+
+rfork en
+
+flagfmt=()
+ptflagfmt='q,w,l library,m machine'
+ptargs='acid options'	# usage: print [acid options] not actual options
+args='system [text]'
+if(! ifs=() eval `{aux/getflags $*} || ~ $#* 0){
+	aux/usage
+	exit usage
+}
+sys=$1
+text=$2
+
+server=`{consoleof $sys}
+if(~ $server ''){
+	echo >[1=2] rdb: unknown console server for $sys
+	exit noconsole
+}
+
+if (~ $text ''){
+	pxe=`{pxeof $sys}
+	if (~ $pxe ''){
+		echo >[1=2] rdb: unknown pxe configuration for $sys
+		exit notext
+	}
+	text=`{grep '^bootfile=' $pxe | sed 's/\.gz$//' | awk -F! '{print $2}'}
+}
+if(! test -f $text){
+	echo >[1=2] rdb: file not found: $text
+	exit notext
+}
+
+# kernel may be installed.  find the original text.
+textbase=`{basename -d $text}
+s=`{src -n $textbase}
+if(~ $#s 1)
+	textbase=`{basename -d $s}
+textbase=`{basename -d $text}
+acid=$textbase/acid
+if(! test -f $acid)
+	@{cd $textbase && mk acid}
+if(! test -f $acid)
+	exit noacidlib
+
+import $server /mnt/consoles
+rdbfs /mnt/consoles/$sys
+acid -k -l kernel -l mach -l $acid $ptflag -r $text
+exit ''