Fix mbnambuf support for multi-byte characters.  If a substring is larger than WIN_CHARS bytes, we shift the suffix (previous substrings) upwards by the amount this substring exceeds its WIN_CHARS slot.  Profiling shows this change is indistinguishable from the previous code at 95% confidence. This bug would result in attempts to access or create files or directories with multi-byte characters returning an error but no data loss.
Fix mbnambuf support for multi-byte characters.  If a substring is larger
than WIN_CHARS bytes, we shift the suffix (previous substrings) upwards
by the amount this substring exceeds its WIN_CHARS slot.  Profiling shows
this change is indistinguishable from the previous code at 95% confidence.
This bug would result in attempts to access or create files or directories
with multi-byte characters returning an error but no data loss.

Reported and tested by:	avatar
MFC after:	3 days

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