mquote: make sed expression to strip signature work with busybox sed

Closes: #74 [via git-merge-pr]

mquote

@@ -7,6 +7,6 @@
 
 printf '%s wrote:\n' "$from"
 mshow -R "$1" |
-	sed -n '/^-- $/!p;//q' |                   # strip signature
+	sed -n '/^-- $/,$!p'   |                   # strip signature
 	sed -e :a -e '/^\n*$/{$d;N;ba' -e '}' |    # strip empty lines
 	sed 's/^/> /'                              # prefix with >