make -j 104 tinderbox has over half of the calls to read find an empty pipe. As the pipe is shared, this contends a lot.
```
commit d7cfa63d1de31e176e50562cdb57efa596792d93 (HEAD, mjgzoo/pipenb)
Author: Mateusz Guzik <mjg@FreeBSD.org>
Date: Mon Aug 8 18:02:15 2022 +0000
pipe: try to skip locking the pipe of a non-blocking fd is used
Reviewed by:
Differential Revision:
commit a5ee622427f86017939bc9ef47e5a7c804072825
Author: Mateusz Guzik <mjg@FreeBSD.org>
Date: Mon Aug 8 19:14:09 2022 +0000
mac: cheaper check for mac_pipe_check_read
Reviewed by:
Differential Revision:
```