Cleanup SP code for packages

In order to make the patch for Oracle-style packages smaller, we'll do the following changes as a separate commit:

• Add Sp_handler::db_find_and_cache_routine(), to avoid duplicate combinations of db_find_routine() followed by sp_cache_insert()
• Add append_suid() and append_comment(), helper functions to share between show for CREATE PROCEDURE and show for CREATE PACKAGE
• Move the code responsible to clone recursive SP routines from Sp_handler::sp_find_routine() into a new method Sp_handler::sp_clone_and_link_routine()
• Add "const" qualifier to LEX_CSTRING parameters to LEX::make_sp_name()
• Add LEX::set_user_variable(), to reduce code duplication in the grammar creating Item_func_set_user_var's
• Add LEX::call_statement_start(), to reduce code duplication in the grammar for "call:"
• Add LEX::add_grant_command(), to reduce code duplication in the grammar for "revoke_command:" and "grant_command:"