Improve user control over truncation of logged bind-parameter values.

 Improve user control over truncation of logged bind-parameter values.
 
 This patch replaces the boolean GUC log_parameters_on_error introduced
 by commit ba79cb5dc with an integer log_parameter_max_length_on_error,
 adding the ability to specify how many bytes to trim each logged
 parameter value to.  (The previous coding hard-wired that choice at
 64 bytes.)
 
 In addition, add a new parameter log_parameter_max_length that provides
 similar control over truncation of query parameters that are logged in
 response to statement-logging options, as opposed to errors.  Previous
 releases always logged such parameters in full, possibly causing log
 bloat.
 
 For backwards compatibility with prior releases,
 log_parameter_max_length defaults to -1 (log in full), while
 log_parameter_max_length_on_error defaults to 0 (no logging).
 
 Per discussion, log_parameter_max_length is SUSET since the DBA should
 control routine logging behavior, but log_parameter_max_length_on_error
 is USERSET because it also affects errcontext data sent back to the
 client.
 
 Alexey Bashtanov, editorialized a little by me
 
 Discussion: https://postgr.es/m/b10493cc-a399-a03a-67c7-068f2791ee50@imap.cc