Process a miscellaneous key sequence in Isearch mode.
Try to convert the current key-sequence to something usable in Isearch
mode, either by converting it with `function-key-map', downcasing a
key with C-
it. (In the last case, we may have to read more events.) If so,
either unread the converted sequence or execute the command.
Otherwise, if `search-exit-option' is non-nil (the default) unread the
key-sequence and exit the search normally. If it is the symbol
`edit', the search string is edited in the minibuffer and the meta
character is unread so that it applies to editing the string.
ARG is the prefix argument. It will be transmitted through to the
scrolling command or to the command whose key-sequence exits Isearch mode.