Reworking zsock_t integration

The zmsg, zframe, zstr, zloop, and zpoller classes now work both with
zsock_t * sockets and older void * sockets, using a runtime probe that
determines the class type.

This means we can gradually move away from the old style to the new
style without breaking old apps, nor needing to duplicate lots of code.

The zsock class no longer uses contexts. I'd like to deprecate the ctx
class and manage contexts automatically:

- one global context per process
- one thread local context per thread

This will also require reworking the zthread class, which was on my list
for some time (it's not consistent with CLASS).