Check PCNTL functions for signal support instead of PCNTL extension
Check PCNTL functions for signal support instead of PCNTL extension

This helps avoiding problems when the PCNTL extension is loaded, but
(some of) its functions are disabled via PHPs `disable_function`
configuration. This is particularly common for the PCNTL functions in
CGI environments, e.g. they are disabled by default on Debian- /
Ubuntu-based distributions. Previously, even when not attaching any
signal listeners, the loop would print a warning on each loop tick:

> PHP Warning:  pcntl_signal_dispatch() has been disabled for security
reasons in {file} on {line}
clue-labs/event-loop,reactphp/event-loop