While working at a client's site on their RH 6.7 boxes, I ran into problems like this one reported on the Support Desk.
I was able to adapt the code so that it at least works and will submit a patch for review.
pnctl was introduced with https://gerrit.wikimedia.org/r/213586 in May 2015, making it part of 1.25 and 1.26 released versions of Scribunto.
Change 273173 had a related patch set uploaded (by MarkAHershberger):
Provide an alternative to the pcntl_* functions
@MarkAHershberger FYI, there are also some people running into this exact same problem on the MediaWiki helpdesk page and it seems that Windows also doesn't support pcntl at all.
Change 336266 had a related patch set uploaded (by Anomie):
LuaStandalone: Fix signal handling
Change 273173 abandoned by Anomie:
Provide an alternative to the pcntl_* functions
Reason:
The reason this hasn't been merged yet is because no one finished testing it. I did so and discovered that not only does this not work correctly, I252ec046 that it's trying to fix was broken too. I submitted I8a62e1660 to fix it all.
Change 336266 merged by jenkins-bot:
[mediawiki/extensions/Scribunto] LuaStandalone: Fix signal handling