because the name "MJIT" is an internal code name, it's inconsistent with
--jit while they are related to each other, and I want to discourage future
JIT implementation-specific (e.g. MJIT-specific) APIs by this rename.
[Feature #17490]
`(ulimit -n 30; make test-tool)` fails with unexplicit message:
"undefined method `write' for nil:NilClass" due to lack of stdout.
This change makes it explicit. [Bug #5577]