MDEV-4928 Merge collation customization improvements

Merging the following MySQL-5.6 changes:
- WL#5624: Collation customization improvements
  http://dev.mysql.com/worklog/task/?id=5624

- WL#4013: Unicode german2 collation
  http://dev.mysql.com/worklog/task/?id=4013

- Bug#62429 XML: ExtractValue, UpdateXML max arg length 127 chars
  http://bugs.mysql.com/bug.php?id=62429
  (required by WL#5624)

sql/sql_class.h

@@ -745,6 +745,35 @@ typedef struct system_status_var
 
 void mark_transaction_to_rollback(THD *thd, bool all);
 
+
+/**
+  Get collation by name, send error to client on failure.
+  @param name     Collation name
+  @param name_cs  Character set of the name string
+  @return
+  @retval         NULL on error
+  @retval         Pointter to CHARSET_INFO with the given name on success
+*/
+inline CHARSET_INFO *
+mysqld_collation_get_by_name(const char *name,
+                             CHARSET_INFO *name_cs= system_charset_info)
+{
+  CHARSET_INFO *cs;
+  MY_CHARSET_LOADER loader;
+  my_charset_loader_init_mysys(&loader);
+  if (!(cs= my_collation_get_by_name(&loader, name, MYF(0))))
+  {
+    ErrConvString err(name, name_cs);
+    my_error(ER_UNKNOWN_COLLATION, MYF(0), err.ptr());
+    if (loader.error[0])
+      push_warning_printf(current_thd,
+                          Sql_condition::WARN_LEVEL_WARN,
+                          ER_UNKNOWN_COLLATION, "%s", loader.error);
+  }
+  return cs;
+}
+
+
 #ifdef MYSQL_SERVER
 
 void free_tmp_table(THD *thd, TABLE *entry);