make handling of ZMTP greetings more RFC compliant

Even though the greeting looks like a normal frame construct, it
is special.

* IDFlags has to be zero
* invalid frames (according to the version supported) are treated
  as valid to enable version negotiation

Move the greeting decoding and handling into dedicated methods
rename gen_zmq to ezmq

gen_zmq is a to generic name for zeromq community project. So in
preperation of becomming one, rename it to ezmq.