transport_check_fds and transport_read_pdu had almost the same
functionality: reading and validating one pdu at a time.
Now transport_read_pdu reads one pdu from the transport layer and verifies
that the pdu data is valid - as before.
transport_read_pdu also ensures that the stream is sealed and
rewound when the pdu is received completely.
transport_check_fds just uses transport_read_pdu and does *not* do
the verification a second time based on the stream.
Besides the clean up this fixes the following problems:
* transport_read always read 4 bytes. Fast-path input synchronize pdus
are only 3 bytes long. In this case on byte got lost in the stream
buffer which lead to "de-synchronization" of server and
client.
* Size check in tpdu_read_connection_confirm - already read bytes
weren't taken into account.
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