Problem: artificial restriction on binary identities

Applications that use ZMQ_IDENTITY can be trapped by the artificial
restriction on not using a binary zero as first byte. It's specially
nasty on random generated identities, e.g. UUIDs, as the chance of a
binary zero is low, so it will pass 255 out of 256 times.

Solution: remove the restriction.
Whitespace fixes
Use different endpoint for each test step
Merge pull request #1037 from hintjens/master

Problem: artificial restriction on binary identities