check-code: warn to use killdaemons instead of kill `cat PIDFILE`

We have a bunch of tests that still use

    kill `cat hg.pid`

or worse,

    kill `cat hg.pid`; while kill -0 `cat hg.pid`; sleep 0; done

Cleaning these up to use tests/killdaemons.py is non-trivial, so for now
we just add a warning.

--HG--
extra : rebase_source : a57224521cb7c4ca0f1b423e2f1834f529d38f84
check-code: fix platform-specific error code variance

test-check-code-hg.t uses xargs to invoke check-code.py on every file in
'hg manifest'. The return code from xargs varies between BSD xargs and
GNU xargs: BSD will return 1 if any invocation exits with an error code;
GNU xargs will return 123 in this case. This normalizes the exit code
back to 1.