xmalloc: remove delim param from xstrjoin() and add separate xstrjoin3()

This avoids the need for an unused third argument for most xstrjoin()
calls and replaces the cases where it's needed with a more flexible
function. Code generation is the same in both cases, when there are 2
string params and a compile-time known delimiter.

This commit also converts 4 uses of xasprintf() to use xstrjoin*().

See also: https://godbolt.org/z/xsjrhv9b6

notify.c

@@ -77,7 +77,7 @@ write_icon_file(const void *data, size_t data_sz, int *fd, char **filename,
 
     LOG_DBG("wrote icon data to %s", name);
     *filename = xstrdup(name);
-    *symbolic_name = xasprintf("file://%s", *filename);
+    *symbolic_name = xstrjoin("file://", *filename);
     return true;
 }