that it tested the privilege in ::update() whereas it should be
in ::check() (see email from Serg, subject
"Re: bk commit - 4.1 tree (guilhem:1.1706)").
So I add instead a check_func function to sys_var_thd_bit.
I do the same addition to sys_var_thd_ulong, to unify handling
of PSEUDO_THREAD_ID with the one of SQL_LOG_BIN. So class
sys_var_pseudo_thread_id is not needed anymore, removing it.