Don't show PSEUDO_THREAD_ID in SHOW VARIABLES because:
- we don't want people to discover this variable as it could never do good to
set it (it was designed for use by mysqlbinlog only, so that a thread can have
several temp tables of the same name at the same time)
- if we show it in SHOW VARIABLES, Mysql Administrator will display it and
this will force us to put a description, so all MySQL Administrator user
will be aware of this variable, some may have the idea to set it with a SET
command, and then it will cause bad things.
The variable is still settable, and still visible with SELECT @@.