helper/command: rename s/command_unknown/jim_command_dispatch/

The function's name was consistent with its purpose to handle
commands that were not at root level, thus not directly 'known'
by jimtcl.

Rename it as jim_command_dispatch() to highlight that now it is
a jim_handler and its purpose is to dispatch the call to the
proper command handler.

Change-Id: I9491a6d6459b8eb37a6c402abcae08388c693764
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5791
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>