tty/teken: fix UTF8 sequence validation logic

This patch fixes UTF-8 sequence validation logic in
teken_utf8_bytes_to_codepoint() and fixes fallback behaviour in
ttydisc_rubchar() when an invalid UTF8 sequence is encountered. The code
previously used __bitcount() to extract sequence length information from
the leading byte. However, this assumption breaks for certain code
points that have additional bits set in the first half of the leading
byte (e.g. Cyrillic characters). This lead to incorrect behaviour when
deleting those characters using backspaces. The code now checks the
number of consecutive set bits in the leading byte starting from the
MSB, as per RFC 3629.

Reviewed by: christos
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D42147