Problem: pipe endpoints were badly generated

Random number overflow caused systematic duplicates.
Solution is to stick to 4-byte values and use two of them.
Bunch of fixes to make zactor/zsock work better

- moved signal handling from zctx to zsys
- interrupt flag is now called zsys_interrupted
- zctx_interrupted is deprecated
- zsys_handler_set/reset methods are removed
- zsys internal methods are explicitly marked in API
- zsock now tracks sockets to allow leak detection
- un-destroyed sockets are reported and then destroyed
- zsock_wait/signal take void * arguments including zactor_t
- zsock_wait filters & discards non-signal messages
Merge pull request #483 from hintjens/master

Improvements to support zactor/zsock