pgsql: Add a pg_encoding_mbcliplen() function that is just like

Log Message:
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Add a pg_encoding_mbcliplen() function that is just like pg_mbcliplen()
except the caller can specify the encoding to work in; this will be needed
for pg_stat_statements. In passing, do some marginal efficiency hacking
and clean up some comments. Also, prevent the single-byte-encoding code
path from fetching one byte past the stated length of the string (this
last is a bug that might need to be back-patched at some point).

Modified Files:
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pgsql/src/backend/utils/mb:
mbutils.c (r1.75 -> r1.76)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/mbutils.c?r1=1.75&r2=1.76)
pgsql/src/include/mb:
pg_wchar.h (r1.81 -> r1.82)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/mb/pg_wchar.h?r1=1.81&r2=1.82)