Minor improvements to mbregress.sh script.

1. Use new dropdb --if-exists option, to avoid alarming the user if
   the database being dropped doesn't already exist.
2. Bail out if createdb fails.
3. exit 1 if the checks fail.
4. Make it executable.

Josh Kupershmidt, with some kibitzing by me.

src/test/mb/mbregress.sh

@@ -14,11 +14,13 @@ if [ ! -d results ];then
     mkdir results
 fi
 
-dropdb utf8
-createdb -T template0 -l C -E UTF8 utf8
+dropdb --if-exists utf8
+createdb -T template0 -l C -E UTF8 utf8 || exit 1
 
 PSQL="psql -n -e -q"
 tests="euc_jp sjis euc_kr euc_cn euc_tw big5 utf8 mule_internal"
+EXITCODE=0
+
 unset PGCLIENTENCODING
 for i in $tests
 do
@@ -54,7 +56,10 @@ do
 		echo "----------------------"; \
 		echo "" ) >> regression.diffs
 		echo failed
+		EXITCODE=1
 	else
 		echo ok
 	fi
 done
+
+exit $EXITCODE