Portability fixes


mysql-test/r/rpl_relayrotate.result:
  Portability fix
mysql-test/t/rpl_relayrotate.test:
  Portability fix.
sql/log_event.cc:
  Removed ++ from uint4korr() becasue this caused side effects
sql/uniques.cc:
  Removed not used argument to get_merge_buffers_cost()
  Removed wrong cast which caused compiler failures
  Cast arguments to ceil() to double (portability problem)