mbedtls: fix handling of blocked sends

mbedtls is picky when a mbedtls_ssl_write) was previously blocked. It
requires to be called with the same amount of bytes again, or it will
lose bytes, e.g. reporting all was sent but they were not. Remember the
blocked length and use that when set.

Reported-by: Tamás Bálint Misius
Fixes #15801
Closes #15846

tests/http/test_08_caddy.py

@@ -212,6 +212,8 @@ class TestCaddy:
 
     @pytest.mark.parametrize("proto", ['http/1.1', 'h2', 'h3'])
     def test_08_08_earlydata(self, env: Env, httpd, caddy, proto):
+        if proto == 'h3' and not env.have_h3():
+            pytest.skip("h3 not supported")
         count = 2
         docname = 'data10k.data'
         url = f'https://{env.domain1}:{caddy.port}/{docname}'