libpq: Add suppress argument to pqTraceOutputNchar

libpq: Add suppress argument to pqTraceOutputNchar

In future commits we're going to trace authentication related messages.
Some of these messages contain challenge bytes as part of a
challenge-response flow.  Since these bytes are different for every
connection, we want to normalize them when the PQTRACE_REGRESS_MODE
trace flag is set.  This commit modifies pqTraceOutputNchar to take a
suppress argument, which makes it possible to do so.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://postgr.es/m/CAGECzQSoPHtZ4xe0raJ6FYSEiPPS+YWXBhOGo+Y1YecLgknF3g@mail.gmail.com