telnet: allow hiding selected commands during auto-completion

telnet: allow hiding selected commands during auto-completion

We have TCL procedure and commands that we do not want to show in
the list of auto-completion. E.g. TCL wrappers for deprecated
commands, internal procedures that are not supposed to be exposed
to user, or even commands that the user decides to hide.

Create a TCL procedure to be called by telnet auto-complete code
in place of the hard-coded TCL command. The procedure will run the
same command and will filter-out the unwanted command names.

Initialize the list of commands to be filtered-out with the name
of the TCL procedure above, as it is considered as internal.

Change-Id: I2d83bbf8194502368c589c85cccb617e69128c69
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6194
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>