tree-wide: get rid of strappend()

It's a special case of strjoin(), so no need to keep both. In particular
as typing strjoin() is even shoert than strappend().
diff --git a/src/tmpfiles/tmpfiles.c b/src/tmpfiles/tmpfiles.c
index 90ba32e..3c30612 100644
--- a/src/tmpfiles/tmpfiles.c
+++ b/src/tmpfiles/tmpfiles.c
@@ -2597,7 +2597,7 @@
 
         case CREATE_SYMLINK:
                 if (!i.argument) {
-                        i.argument = path_join("/usr/share/factory/", i.path);
+                        i.argument = path_join("/usr/share/factory", i.path);
                         if (!i.argument)
                                 return log_oom();
                 }
@@ -2613,7 +2613,7 @@
 
         case COPY_FILES:
                 if (!i.argument) {
-                        i.argument = path_join(arg_root, "/usr/share/factory/", i.path);
+                        i.argument = path_join(arg_root, "/usr/share/factory", i.path);
                         if (!i.argument)
                                 return log_oom();