Replace removed config options from 5.5 and old 5.6 installations

The two options key-buffer and myisam-recover were replaced in 5.5
with key-buffer-size and myisam-recover-options. In 5.7 the unique-prefix
option that allowed these to work in 5.6 was also removed. If the user
has customized the old config file it will not be replaced and the server
will fail to start.
Renaming these two options in /etc/mysql/my.cnf should clear up a common
upgrade error.
If the config file is changed, we create a backup in
/etc/mysql/my.cnf.migrated.bak (LP: #1571865)

debian/mysql-server-5.7.postinst

@@ -63,6 +63,18 @@ test_mysqld_startup() {
   return $result
 }
 
+# Default config in 5.5 and older 5.6 installations contains two deprecated
+# options that were removed in 5.7. They were renamed, so we can fix this
+# automatically. We create a backup of the old file
+fix_old_config_options() {
+  sed -e 's/key_buffer[[:space:]]*=/# Obsolete key_buffer option renamed to key_buffer_size by maintainer script\nkey_buffer_size\t\t=/' \
+      -e 's/myisam-recover[[:space:]]*=/# Obsolete myisam-recover option renamed to myisam_recover_options by maintainer script\nmyisam_recover_options\t=/' /etc/mysql/my.cnf.migrated --in-place=.bak
+  # If nothing was changed, remove the new file
+  if cmp -s /etc/mysql/my.cnf.migrated /etc/mysql/my.cnf.migrated.bak; then
+    mv /etc/mysql/my.cnf.migrated.bak /etc/mysql/my.cnf.migrated
+  fi
+}
+
 # Check if there is passwordless root login
 test_mysql_access() {
   mysql --no-defaults -u root -h localhost </dev/null >/dev/null 2>&1
@@ -155,6 +167,12 @@ case "$1" in
     # data directory and then somewhen gets purged by the admin.
     db_set mysql-server/postrm_remove_database false || true
 
+    # Fix old options that were deprecated in 5.5 and removed in 5.7
+    if dpkg --compare-versions "$2" le-nl "5.7.12-1ubuntu1~"; then
+      echo "Renaming removed key_buffer and myisam-recover options (if present)"
+      fix_old_config_options
+    fi
+
     # Sanity check to make sure the server can start
     test_mysqld_startup