Implemented IF-THEN-ELSE.


sql/sp_head.cc:
  Reimplemented the broken backpatching, so it works for nested constructions and
  IF-THEN-ELSE too.
sql/sp_head.h:
  Reimplemented the broken backpatching, so it works for nested constructions and
  IF-THEN-ELSE too.
sql/sp_pcontext.cc:
  Return the value from push-methods, for convenience.
sql/sp_pcontext.h:
  Return the value from push-methods, for convenience.
sql/sql_yacc.yy:
  Implemented IF-THEN-ELSE.
  Corrected for the new backpatch method.