Add loop mode (-L) in the CLI SAPI

[mnapoli]

This is a pull request that I opened in my own fork to preview the diff and allow code review.

This PR adds a new "Loop" mode that can be used with -L. For example: php -L script.php.

This mode runs the script in a loop.

The big difference with doing a while(true) {} loop in PHP is that every execution of the script is isolated from the other executions.

Use cases

• Workers, i.e. processing jobs in a message queue: instead of running in a loop the script can process one job, the "Loop" mode will ensure the script will loop => each job will be processed in a fresh PHP environment, meaning no memory leaks, no shared state, etc.
• AWS Lambda: by running php -L each event/request would be processed in an isolated PHP environment without having the overhead of starting a new process (this is the main reason that motivated this work, I will publish more information about this soon)

Maybe there are opportunities too for ReactPHP/Amp/Aerys/etc to benefit from that (provide very fast response times without having to sacrifice request isolation).

Example

Before:

<?php
// script.php
$a = 'foo';
while (true) {
    echo $a . PHP_EOL;
    $a = 'bar';
}

$ php -L script.php
foo
bar
bar
[...]

With the "loop mode":

<?php
// script.php
$a = 'foo';
echo $a . PHP_EOL;
$a = 'bar';

$ php -L script.php
foo
foo
foo
[...]

[mnapoli]

Replaced by #2 to be on the 7.2 branch.