transport refactor

rename transport_read to transport_read_pdu. This name is more
descriptive what the function actually does.
refactor transport_read_pdu and check_fds

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.
fix: typos and formating
Merge pull request #1982 from bmiklautz/feat/read_pdu

Transport re-factor and clean up