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().

src/debug-generator/debug-generator.c

@@ -131,7 +131,7 @@ static int generate_wants_symlinks(void) {
                 if (!p)
                         return log_oom();
 
-                f = strappend(SYSTEM_DATA_UNIT_PATH "/", *u);
+                f = path_join(SYSTEM_DATA_UNIT_PATH, *u);
                 if (!f)
                         return log_oom();