Avoid using LEX::spname during CREATE PROCEDURE and CREATE FUNCTION

The code in sp_create_routine() uses two ways to access the routine name:

• Via sp_head: sp->m_db and sp->m_name
• Via LEX: lex->spname

This is a fragment from sp_create_routine():

  if (!(table= open_proc_table_for_update(thd)))

  {

    my_error(ER_SP_STORE_FAILED, MYF(0), SP_TYPE_STRING(type),sp->m_name.str)

    goto done;

  }

  else

  {   

    /* Checking if the routine already exists */

    if (db_find_routine_aux(thd, type, lex->spname, table) == SP_OK)

    {

The function mysql_create_routine() uses the same style:

  if (check_db_name((LEX_STRING*) &lex->sphead->m_db))  

  {

  ...

    if (lex->create_info.or_replace())

  {

    if (check_routine_access(thd, ALTER_PROC_ACL, lex->spname->m_db.str,

  ...

In the above code, both LEX::sp_name and sp_sphead point to copies of the same qualified routine name. Copying is done in sql_yacc.yy:

             if (!Lex->make_sp_head_no_recursive(thd, $1, $2,

                                                 TYPE_ENUM_FUNCTION))

               MYSQL_YYABORT;

             Lex->spname= $2;

We're going to reuse sp_head to store Oracle-style packages soon (see MDEV-10591).
To avoid duplicating of this redundancy, we should get rid of it before implementing packages.

Under terms of this task we will:
1. Fix the code responsible for CREATE PROCEDURE and CREATE FUNCTION not to use lex->spname, and to use sphead instead. This includes functions:

• mysql_create_routine()
• sp_create_routine()

2. Remove copying of the routine name to LEX::spname. The latter should stay NULL during CREATE PROCEDURE and CREATE FUNCTION.