The patch contains the following changes:
  - Introduce auxilary functions to convenient work with character sets:
    - resolve_charset();
    - resolve_collation();
    - get_default_db_collation();
  - Introduce lex_string_set();
  - Refactor Table_trigger_list::process_triggers() &
    sp_head::execute_trigger() to be consistent with other code;
  - Move reusable code from add_table_for_trigger() into
    build_trn_path(), check_trn_exists() and load_table_name_for_trigger()
    to be used in the following patch.
  - Rename triggers_file_ext and trigname_file_ext into TRN_EXT and
    TRG_EXT respectively.


include/my_sys.h:
  Introduced auxilary functions (to be used in the following patch).
mysys/charset.c:
  Introduced auxilary functions (to be used in the following patch).
sql/handler.cc:
  Rename triggers_file_ext -> TRG_EXT;
  Rename trigname_file_ext -> TRN_EXT.
sql/mysql_priv.h:
  1. Fix typo;
  2. Introduce auxilary functions (set_lex_string() will  be used
     in the following patch);
  3. Rename triggers_file_ext -> TRG_EXT;
     Rename trigname_file_ext -> TRN_EXT.
sql/sp_head.cc:
  Make sp_head::execute_trigger() consistent with
  sp_head::execute_function() and sp_head::execute_procedure().
sql/sp_head.h:
  Make sp_head::execute_trigger() consistent with
  sp_head::execute_function() and sp_head::execute_procedure().
sql/sql_db.cc:
  1. Introduce auxilary function.
  2. Polishing.
sql/sql_trigger.cc:
  1. Move common code from add_table_for_trigger() into
     - build_trn_path();
     - check_trn_exists();
     - load_table_name_for_trigger();
  2. Polishing.
sql/sql_trigger.h:
  1. Move common code from add_table_for_trigger() into
     - build_trn_path();
     - check_trn_exists();
     - load_table_name_for_trigger();
  2. Polishing.