z/OS: Documented SIGPIPE considerations

Updated:
    README.md: Documented need for application to handle/ignore SIGPIPE
z/OS: signal(SIGPIPE, SIG_IGN) for tests

Updated:
   tests/testutil.hpp: Add signal(SIGPIPE, SIG_IGN) to
        setup_test_environment(), on z/OS (__MVS__)
z/OS: Loop on EAGAIN on close() in ~signaler

Updated:
    src/signaler.cpp: Add close_wait_ms() static function to loop
       when receiving EAGAIN in response to close(), with ms long
       sleeps, up to a maximum limit (default 2000ms == 2 seconds);
       used in signaler_t::~signaler_t() destructor.
Merge pull request #1138 from ewen-naos-nz/zos

z/OS UNIX System Services port: EAGAIN, SIGPIPE