Don't use %s-with-precision format spec to truncate data being displayed

Don't use %s-with-precision format spec to truncate data being displayed
in a COPY error message.  It seems that glibc gets indigestion if it is
asked to truncate strings that contain invalid UTF-8 encoding sequences.
vsnprintf will return -1 in such cases, leading to looping and eventual
memory overflow in elog.c.  Instead use our own, more robust pg_mbcliplen
routine.  I believe this problem accounts for several recent reports of
unexpected 'out of memory' errors during COPY IN.