UCS_NO_CHAR here means the input was too short. Because the strings
generally come from the source code or from PO files, they should not
end in the middle of a character. However, the whole character may be
missing if the string is empty. So select_button_by_key() now checks
for that case separately.
UCS_NO_CHAR must not be passed to unicode_fold_label_case() because
the result is undefined.
The configure script no longer recognizes "CONFIG_UTF_8=yes" lines
in custom features.conf files. They will have to be changed to
"CONFIG_UTF8=yes". This incompatibility was deemed acceptable
because no released version of ELinks supports CONFIG_UTF_8.
The --enable-utf-8 option was not renamed.
