After the last refactoring work in preparation and optimization,
there are some unused code blocks in sql_union.cc.
This bug fix eliminates those code blocks completely and performs
some simple additional refactoring.

- Added an interface st_select_lex_unit::is_simple() that wraps
  testing for !(is_union() || fake_select_lex)

- Cleaned up global_parameters() a bit: Only ORDER BY/LIMIT/OFFSET
  should be accessed through it, otherwise use fake_select_lex.

- Initialization of JOIN::do_send_rows was moved from optimization to
  execution, since it is used only in the latter.

- st_select_lex_unit::prepare() has mostly cosmetic changes and improved
  comments. Call to set_current_select() eliminated for error case.

- Deleted unused function Query_result::reset_offset_limit_cnt

- st_select_lex_unit::optimize() had an unused code block started with
  if (sl == global_parameters() && is_union()).
  It was unused because global_parameters() always return the "fake" object
  for a UNION query. Besides, with the introduction of
  Query_result_union_direct, LIMIT/OFFSET handling is correct without
  adjusting offset_limit_cnt and select_limit_cnt.

- An equivalent code block is removed from st_select_lex_unit::execute().
  Variable rows_at_start was found to be redundant.
  Calls to set_current_select() were removed in error case.
  offset_limit_cnt did not need to be assigned here, since it is done
  in set_limit().
  Call info(HA_STATUS_VARIABLE) was moved to a more logical place
  (used to get row count from temporary table used by UNION).
  add_rows was never assigned so it could be removed:
  sl->join->calc_found_rows is never true for a query block that is
  part of a UNION (either braces=true or m_select_limit=HA_POS_ERROR,
  see JOIN::optimize()), search for comment "Calculate found rows if".
  join->examined_rows is reset in JOIN::exec() so assignment is deleted.

- Added more extensive tests for LIMIT and OFFSET to limit.test

(cherry picked from commit b50c7dc66c6c894772d5da463cd08b9cd9ebd154)

Conflicts:
	sql/query_result.h
	sql/sql_union.cc