Check if proc_open() can be used before calling it

In a environment that `proc_open()` is disabled, the `wp` command itself could not show any help text:

<pre># wp
PHP Notice:  Undefined variable: pipes in phar:///usr/local/bin/wp/php/commands/help.php on line 106
PHP Warning:  proc_open() has been disabled for security reasons in phar:///usr/local/bin/wp/php/commands/help.php on line 106
PHP Warning:  proc_close() expects parameter 1 to be resource, null given in phar:///usr/local/bin/wp/php/commands/help.php on line 106</pre>