zmq_ppoll mostly mimics zmq_poll behavior, except for the added feature of being able to specify a signal mask. Signals in this mask will be blocked during execution of zmq_ppoll. Switching of the process' active signal mask happens atomically with the actual poll call, so that no race conditions can occur. This behavior is useful when one wants to gracefully handle POSIX signals without race conditions. See e.g. the discussion below https://250bpm.com/blog:12/ for an explanation.
Also includes two new tests:
1. test_zmq_ppoll_fd does the same thing as test_zmq_poll_fd, demonstrating backwards compatibility with zmq_poll when used with a default signal mask.
2. test_zmq_ppoll_signals demonstrates the use of zmq_ppoll with a signal mask, blocking out SIGTERM everywhere except in zmq_ppoll, allowing to handle the signal in one place without having to worry about race conditions.
When the build is performed with cmake option `-DPOLLER=select`, the following warning is triggered from 4 locations that import `polling_util.hpp`:
```
[...]/src/polling_util.hpp:115:50: warning: unused parameter 'pollset_' [-Wunused-parameter]
inline size_t valid_pollset_bytes (const fd_set &pollset_)
^
1 warning generated.
```
This is fixed here.
