Fix infinite loop when help is bound to a named key combination.

Fix infinite loop when help is bound to a named key combination.

Commit a07e8215a0ef introduced a bug in km_error_key, which is called
when an unbound key is pressed.

If help is bound to a sequence containing named keys (e.g. <esc>), the
raw (untokenized) string would be pushed back into the unget buffer.
This could lead to an infinite loop of unbound key presses triggering
more unbound keys being put into the unget buffer.

Change km_error_key to tokenize the string before putting it in the unget buffer.

Much thanks to Jiri Bohac for his bug report, analysis, and initial patch!