MDEV-5831 Upgrade from MariaDB 5.5.36 via yum fails

Allow "rpm -q --whatprovides mysql-server" to return more than one row.
(all rows should be identical though)

support-files/rpm/server-prein.sh:
  don't use "head -1" or "uniq" to avoid introducing new dependencies.
  "sed" is already used in this script.

support-files/rpm/server-prein.sh

@@ -3,6 +3,7 @@
 
 installed=`rpm -q --whatprovides mysql-server 2> /dev/null`
 if [ $? -eq 0 -a -n "$installed" ]; then
+  installed=`echo "$installed"|sed -n -1p`
   vendor=`rpm -q --queryformat='%{VENDOR}' "$installed" 2>&1`
   version=`rpm -q --queryformat='%{VERSION}' "$installed" 2>&1`
   myvendor='%{mysql_vendor}'