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/basic/cgroup-util.c

@@ -1922,7 +1922,7 @@ char *cg_escape(const char *p) {
         }
 
         if (need_prefix)
-                return strappend("_", p);
+                return strjoin("_", p);
 
         return strdup(p);
 }