Replace
  * select_lex::offset_limit
  * select_lex::select_limit
  * select_lex::explicit_limit
with select_lex::Lex_select_limit

The Lex_select_limit already existed with the same elements and was used in
by the yacc parser.

This commit is in preparation for FETCH FIRST implementation, as it
simplifies a lot of the code.

Additionally, the parser is simplified by making use of the stack to
return Lex_select_limit objects.

Cleanup of init_query() too. Removes explicit_limit= 0 as it's done a bit later
in init_select() with limit_params.empty()