util: rework strappenda(), and rename it strjoina()

After all it is now much more like strjoin() than strappend(). At the
same time, add support for NULL sentinels, even if they are normally not
necessary.

src/sysusers/sysusers.c

@@ -81,7 +81,7 @@ static uid_t search_uid = UID_INVALID;
 static UidRange *uid_range = NULL;
 static unsigned n_uid_range = 0;
 
-#define fix_root(x) (arg_root ? strappenda(arg_root, x) : x)
+#define fix_root(x) (arg_root ? strjoina(arg_root, x) : x)
 
 static int load_user_database(void) {
         _cleanup_fclose_ FILE *f = NULL;
@@ -217,7 +217,7 @@ static int make_backup(const char *target, const char *x) {
         /* Don't fail on chmod() or chown(). If it stays owned by us
          * and/or unreadable by others, then it isn't too bad... */
 
-        backup = strappenda(x, "-");
+        backup = strjoina(x, "-");
 
         /* Copy over the access mask */
         if (fchmod(fileno(dst), st.st_mode & 07777) < 0)