Goal: don't assume uid 0 == "root", use getpwuid to fetch it
Fixes: #81924

Status wrt upstream: It was submitted upstream upstream (3 Jun 2005).
                     Some modification may be needed before its inclusion
                     upstream (multiple uid 0 accounts).
http://lists.alioth.debian.org/pipermail/pkg-shadow-devel/2005-June/001287.html

Index: shadow-4.1.0/src/su.c
===================================================================
--- shadow-4.1.0.orig/src/su.c
+++ shadow-4.1.0/src/su.c
@@ -449,7 +449,12 @@
 			optind++;
 	}
 	if ('\0' == name[0]) {		/* use default user ID */
-		(void) strcpy (name, "root");
+		struct passwd *root_pw = getpwuid(0);
+		if (root_pw == NULL) {
+			SYSLOG((LOG_CRIT, "There is no UID 0 user."));
+			su_failure(tty);
+		}
+		strcpy(name, root_pw->pw_name);
 	}
 
 	doshell = argc == optind;	/* any arguments remaining? */