fix mdoc2man.awk formatting for top-level lists

Reported by Glenn Golden
Diagnosis and fix from Ingo Schwarze
diff --git a/mdoc2man.awk b/mdoc2man.awk
index 80e8d5f..3e87254 100644
--- a/mdoc2man.awk
+++ b/mdoc2man.awk
@@ -321,6 +321,8 @@
       w=nwords
     } else if(match(words[w],"^El$")) {
       optlist=oldoptlist
+      if(!optlist)
+        add(".PP")
     } else if(match(words[w],"^Bk$")) {
       if(match(words[w+1],"-words")) {
 	w++