SocketCAN low level fixes

[felixdivo]

Changes to SocketCAN CTypes:

• libc.write(...) might write less bytes than requested, and is now called in a loop until all data has been transmitted
• better error messages when sending: old can.socketcan.ctypes failed to transmit -> new can.socketcan_ctypes failed to transmit: ENETDOWN (errno 100): Network is down

Changes to SocketCAN Native:

• socket.send(...) might write less bytes than requested, and now socket.sendall(...) is used
• uses similar error messages when sending like SocketCAN CTypes

Other:

• removed the executable flag from test/listener_test.py
• Added a IS_UNIX flag to test/config.py (it was required by a newly created unit test)

[felixdivo]

I tested the ctypes part rudimentary with some examples, but I would like to wait at least until #293 is merged so we can test SocketCAN on Travis CI. It would be good to also write more tests (see #273), maybe we could use GenericInterfaceTest from #290 for that.

[hardbyte]

I'm sure some people would disagree with this definition of UNIX :-P

[felixdivo]

Yeah sure. But it works here and I could not find anything better in the first Google attempt ;-)