pcntl_fork / parent exit() not returning to shell

### HHVM / PHP Versions

$ php --version (works)
PHP 5.5.9-1ubuntu4.25 (cli) (built: May 10 2018 14:37:18)

$ hhvm --version (incompatibility)
HipHop VM 3.26.3 (rel)
$ hh_client --version
hh-b018e564c11827a6aca2edf495e909a17e2b85e5-3.26.3 Fri May 25 06:00:43 2018

$ hhvm --version (old version - works)
HipHop VM 3.15.3 (rel)

### Operating System and Version

Ubuntu 14.04 LTE

### Standalone code, or other way to reproduce the problem

```
$ cat forktest.php

```

### Actual result

Using: HipHop VM 3.26.3 (rel) - latest version available in 14.04 hhvm repo http://dl.hhvm.com/ubuntu
```
$ hhvm forktest.php
forking..
Parent thread - exiting. Should return to bash
<>
Main thread exiting now..
$ <>
```

### Expected result
```
$ php forktest.php
forking..
Parent thread - exiting. Should return to bash
$ <>
$ Main thread exiting now..
```

Using HipHop VM 3.15.3 (rel)
```
$ hhvm forktest.php
forking..
Parent thread - exiting. Should return to bash
$ <>
$ Main thread exiting now..
```

hhvm php.ini file slightly changed between versions in response to package maintainers recommended baseline:
```
$ cd /etc/hhvm; diff php.ini php.ini.dpkg-old
2,4d1
< session.save_handler = files
< session.save_path = /var/lib/hhvm/sessions
< session.gc_maxlifetime = 1440
8a6,8
> ;hhvm.log.runtime_error_reporting_level = 8191
> ;Based on the php cli error reporting level:
> hhvm.log.runtime_error_reporting_level = 22519
```