Add a pg_encoding_mbcliplen() function that is just like pg_mbcliplen()

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).