Enhance return value from pamAuthenticateUser

This enhances the return value from the pamAuthenticateUser function so
callers can articulate PAM error codes like PAM_NEW_AUTHTOK_REQD which
means the credentials are correct, but the password must be changed.

Tested: Yes, scenarios via both Redfish login and Basic Auth:
 - correct username and password, password is not expired
 - correct username and password, password is expired
 - correct username and incorrect password, password is not expired
 - correct username and incorrect password, password is expired
 - non-existent user (passsword is not relevant)

Signed-off-by: Joseph Reynolds <joseph-reynolds@charter.net>
Change-Id: I1114d6c9cc591fb0a1853cb4edea32ad22f7b015
diff --git a/redfish-core/lib/redfish_sessions.hpp b/redfish-core/lib/redfish_sessions.hpp
index 88f250b..0094858 100644
--- a/redfish-core/lib/redfish_sessions.hpp
+++ b/redfish-core/lib/redfish_sessions.hpp
@@ -196,7 +196,7 @@
             return;
         }
 
-        if (!pamAuthenticateUser(username, password))
+        if (pamAuthenticateUser(username, password) != PAM_SUCCESS)
         {
             messages::resourceAtUriUnauthorized(res, std::string(req.url),
                                                 "Invalid username or password");