Add ISABELLE_PID for proof_general.ML

--- a/bin/isabelle-process	Wed Jul 13 16:08:21 2005 +0200
+++ b/bin/isabelle-process	Wed Jul 13 16:32:15 2005 +0200
@@ -202,7 +202,6 @@
 ISABELLE_TMP="$ISABELLE_TMP_PREFIX$$"
 mkdir -p "$ISABELLE_TMP"
 
-
 ## run it!
 
 ML_SYSTEM_BASE=$(echo "$ML_SYSTEM" | cut -f1 -d-)
@@ -218,6 +217,7 @@
 fi
 
 export INFILE OUTFILE COPYDB COMPRESS MLTEXT TERMINATE NOWRITE ISABELLE_TMP
+export ISABELLE_PID="$$"
 
 if [ -f "$ISABELLE_HOME/lib/scripts/run-$ML_SYSTEM" ]; then
   "$ISABELLE_HOME/lib/scripts/run-$ML_SYSTEM"