variable. The problem was that THD::connect_utime could be
used without being initialized when the main thread is used
to handle connections (--thread-handling=no-threads).

sql/mysqld.cc:
  Set THD::start_utime even in no-threads handling mode.
sql/sql_class.cc:
  Initialize variable.
sql/sql_class.h:
  Rename connect_utime to prior_thr_create_utime as to
  better reflect it's use intention.
sql/sql_connect.cc:
  Check only if a thread was actually created.