Previously it was mapped (as a default) to `G_IO_ERROR_FAILED`.

It’s the error that macOS returns when trying to connect to a socket which
is bound but not listened to. Linux returns `ECONNREFUSED` in this case.
It’s helpful if they both map to the same `GIOError` value.

This should fix the `/socket-client/connection-fail` test on macOS,
which is currently
[failing](https://gitlab.gnome.org/GNOME/glib/-/jobs/3970547

) with:
```
 # GLib-GIO-DEBUG: GSocketClient: Starting TCP connection attempt
 # GLib-GIO-DEBUG: GSocketClient: Connection attempt failed: Can't assign requested address
 # GLib-GIO-DEBUG: GSocketClient: Starting new address enumeration
 # GLib-GIO-DEBUG: GSocketClient: Address enumeration completed (out of addresses)
 # GLib-GIO-DEBUG: GSocketClient: Address enumeration failed: (null)
 # GLib-GIO-DEBUG: GSocketClient: Connection failed: Could not connect to localhost: Can't assign requested address
not ok /socket-client/connection-fail - GLib-GIO:ERROR:../gio/tests/gsocketclient-slow.c:231:test_connection_failed: assertion failed (local_error == (g-io-error-quark, 39)): Could not connect to localhost: Can't assign requested address (g-io-error-quark, 0)
Bail out!
```

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

See: #3184
Fixes: #3394