Goal: Retrieve the PAM username in case a module changed the PAM_USER
      item.

According to Linux-PAM_ADG:
 * Note, modules can change the values of PAM_USER and PAM_RUSER during
   any of the pam_*() library calls. For this reason, the application
   should take care to use the pam_get_item() every time it wishes to
   establish who the authenticated user is (or will currently be).

PAM_USER description:

    The username of the entity under whose identity service will be given. That
    is, following authentication, PAM_USER identifies the local entity that
    gets to use the service. Note, this value can be mapped from something
    (eg., "anonymous") to something else (eg. "guest119") by any module in the
    PAM stack. As such an application should consult the value of PAM_USER
    after each call to a PAM function.

See also: https://www.redhat.com/archives/pam-list/2008-May/msg00009.html

Index: shadow-4.1.0/src/su.c
===================================================================
--- shadow-4.1.0.orig/src/su.c
+++ shadow-4.1.0/src/su.c
@@ -309,6 +309,8 @@
 	char **envp = environ;
 	char *shellstr = NULL;
 	char *command = NULL;
+	char *tmp_name;
+	char **ptr_tmp_name = &tmp_name;
 
 #ifdef USE_PAM
 	char **envcp;
@@ -674,6 +676,14 @@
 			su_failure (tty);
 		}
 	}
+	ret = pam_get_item(pamh, PAM_USER, (const void **) ptr_tmp_name);
+	if (ret != PAM_SUCCESS) {
+		SYSLOG((LOG_ERR, "pam_get_item: internal PAM error\n"));
+		fprintf(stderr, "%s: Internal PAM error retrieving username\n", Prog);
+		(void) pam_end(pamh, ret);
+		su_failure(tty);
+	}
+	strncpy(name, tmp_name, sizeof(name) - 1);
 #else				/* !USE_PAM */
 	/*
 	 * Set up a signal handler in case the user types QUIT.