unistr/u8-mbtouc: Improve handling of ill-formed UTF-8 input.

unistr/u8-mbtouc: Improve handling of ill-formed UTF-8 input.

* lib/unistr/u8-mbtouc.c (u8_mbtouc): For an invalid multibyte
character, return the number of bytes that belong together, not always
1.
* lib/unistr/u8-mbtouc-unsafe.c (u8_mbtouc_unsafe): Likewise.
* lib/unistr/u8-mbtouc-aux.c (u8_mbtouc_aux): Likewise.
* lib/unistr/u8-mbtouc-unsafe-aux.c (u8_mbtouc_unsafe_aux): Likewise.
* lib/unistr/u8-mbsnlen.c (u8_mbsnlen): Use u8_mbtouc to determine the
number of bytes of an invalid character.
* tests/unistr/test-u8-mbtouc.c (test_safe_function): New function.
(main): Invoke it.
* tests/unistr/test-u8-mbtouc.h (test_function): Update two test results.
* tests/unistr/test-u8-mbsnlen.c (main): Test various kinds of
malformed byte sequences.
* modules/unistr/u8-mbtouc (configure.ac): Bump version number.
* modules/unistr/u8-mbtouc-unsafe (configure.ac): Likewise.
* modules/unistr/u8-mbsnlen (configure.ac): Likewise.
Reported by Ben Pfaff and Paolo Bonzini.