mbtowc(3): set errno to EILSEQ if an incomplete character is passed.

According to POSIX, The mbtowc() function shall fail if:
[EILSEQ] An invalid character sequence is detected.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D5496

Obtained from: OpenBSD (Ingo Schwarze)
MFC after: 1 month