Updated man pages
Problem: need condition string send

Often we have class property strings that are null, if not set. To send these
safely needs additional logic.

Solution: zstr_send() accepts NULL string as argument and sends as "". Before,
this would assert, so there is no backwards compatibility issue.
Problem: zactor_destroy blocks forever when actor is already dead

This can happen when actor threads terminate for their own reasons.
The zactor_destroy waits forever on zstr_send () as there is no-one
at the other end of the pipe.

Solution: don't block on sending, and if sending failed, do not wait
for a signal from the actor thread.
Problem: zuuid_set_str generated empty uuid string

Although hex decoding gave correct binary value, the conversion
overwrote self->str with null bytes, causing this to be empty.

Solution: added check for this in selftest and fixed the code.
Merge pull request #569 from hintjens/master

Small fixes needed for Zyre gossip work