Generating strings for XKB data used a single shared static buffer,
which offered several opportunities for errors. Use a ring of
resizable buffers instead, to avoid problems when strings end up
longer than anticipated.
XkbStringText escapes non-printable characters using octal numbers.
Such escape sequence would be at most 5 characters long ("\0123"), so
it reserves 5 bytes in the buffer. Due to char->unsigned int
conversion, it would print much longer string for negative numbers.
