support old watchman daemons
Summary:
This supports older watchman daemons, such as the one included in Ubuntu 20.04
on ubuntu:
```
$ watchman get-sockname
{
"version": "4.9.0",
"sockname": "/tmp/ubuntu-state/sock"
}
```
on fb:
```
{
"version": "
20211101",
"sockname": "/opt/facebook/watchman/var/run/watchman/fredemmott-state/sock",
"unix_domain": "/opt/facebook/watchman/var/run/watchman/fredemmott-state/sock"
}
```
D32374969 for changing watchman itself in future releases; this patch is applied to the pinned version we use in open source builds of hhvm.
Reviewed By: fanzeyi
Differential Revision:
D32376204
fbshipit-source-id:
04bb2ef75bb00d177525382629ab158f0a432d77