repeat_prefix keymap and repeat_num_auto UI option

repeat_prefix in the keymap submenu can now be set as an explicit key to
show the command repeat count prompt, e.g. 'n' for 'n20s'.

The repeat_num_auto interface option controls if pressing a number key
automatically shows the command repeat count prompt (true, default) or
not (false).  Number keys can then be keymapped to commands if desired.