Add a new ereport auxiliary function errdetail_log(), which works the same as

Add a new ereport auxiliary function errdetail_log(), which works the same as
errdetail except the string goes only to the server log, replacing the normal
errdetail there.  This provides a reasonably clean way of dealing with error
details that are too security-sensitive or too bulky to send to the client.

This commit just adds the infrastructure --- actual uses to follow.