telnet: auto-completion of "registered" commands

telnet: auto-completion of "registered" commands

auto-completion behavior:
- if there is only one matched command complete the user-command
- else if multiple matches add the common part then in second step
  list all matched commands
- sub-commands are handled in the same way
- auto-completion restarts after these characters ';', '[', '{'

Change-Id: I1b81dd19191a5785e68d0bb5cd244e01a4dd0587
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/6095
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>