Call pcntl_waitpid() with WNOHANG option

• Call pcntl_waitpid() with WNOHANG option when no children spawned.

EXPERIMENTAL 0.2.0

• Experimental release, do not use this release.
• Prevent to blocking at pcntl_waitpid() for long time with PHP 7.
• Set WNOHANG at all times and sleep 1 second when no child processes spawned.