Fix incorrect error message reported for non-existent users

Bruce Momjian <bruce@momjian.us>

Commit: 613c6d26bd42dd8c2dd0664315be9551475b8864
Author: Bruce Momjian <bruce@momjian.us>
Date: 2013-12-18T17:16:21Z
Releases: 9.4.0
Fix incorrect error message reported for non-existent users

Previously, lookups of non-existent user names could return "Success";
it will now return "User does not exist" by resetting errno.  This also
centralizes the user name lookup code in libpgport.

Report and analysis by Nicolas Marchildon;  patch by me

Files