In the previous version, invalid UTF-8 from a terminal caused
UCS_NO_CHAR (0xFFFFFFFD) to be stored in a term_event_key_T, resulting
in -3 which was then incidentally treated as an unassigned special key.
Now, invalid UTF-8 is instead mapped to UCS_REPLACEMENT_CHARACTER
and treated as a character. The fact that handle_interlink_event
calls term_send_ucs when it receives invalid UTF-8 makes it pretty
clear that this is how it was intended.
src/viewer/text/link.c (not changed in this commit) already referred
to UCS_REPLACEMENT_CHARACTER in a comment even though it was not
previously defined.
It cannot be restricted just to characters that have passed
check_kbd_label_key(), because hotkeys in strings received from
gettext must also be processed with it, and there we don't have
a struct term_event for check_kbd_label_key().
This requires compiling cp2u() in even without CONFIG_UTF_8.
I also added an is_kbd_character macro to make try_document_key
more resilient to changes in the definition of term_event_key_T.
