Log that it's xrootd-multiuser denying access, not the file system

Since the various operations just return -EACCES when xrootd-multiuser
couldn't change users (or groups), it's confusing whether access is denied
by the file system, or by xrootd-multiuser itself.  Make it explicit
via a log message.

Author: matyasselmeci

src/UserSentry.hh

@@ -153,17 +153,21 @@ public:
         if (result == nullptr) {
             if (retval) {  // There's an actual error in the lookup.
                 m_log.Emsg("UserSentry", "Failure when looking up UID for username", username.c_str(), strerror(retval));
+                m_log.Emsg("UserSentry", "Multiuser denying access");
             } else {  // Username doesn't exist.
                 m_log.Emsg("UserSentry", "XRootD mapped request to username that does not exist:", username.c_str());
+                m_log.Emsg("UserSentry", "Multiuser denying access");
             }
             return;
         }
         if (pwd.pw_uid < g_minimum_uid) {
             m_log.Emsg("UserSentry", "Username", username.c_str(), "maps to a system UID; rejecting lookup");
+            m_log.Emsg("UserSentry", "Multiuser denying access");
             return;
         }
         if (pwd.pw_gid < g_minimum_gid) {
             m_log.Emsg("UserSentry", "Username", username.c_str(), "maps to a system GID; rejecting lookup");
+            m_log.Emsg("UserSentry", "Multiuser denying access");
             return;
         }
 
@@ -182,6 +186,7 @@ public:
         } while (1);
         if (-1 == retval) {
             m_log.Emsg("UserSentry", "Failure when looking up supplementary groups for username", username.c_str());
+            m_log.Emsg("UserSentry", "Multiuser denying access");
             return;
         }
 
@@ -193,6 +198,7 @@ public:
         m_orig_uid = setfsuid(result->pw_uid);
         if (m_orig_uid < 0) {
             m_log.Emsg("UserSentry", "Failed to switch FS uid for user", username.c_str());
+            m_log.Emsg("UserSentry", "Multiuser denying access");
             return;
         }
         m_orig_gid = setfsgid(result->pw_gid);