Summary:
Don't call postLogMsg to post a user msg, because it truncates messages to
512 bytes.

Rename traceCap_stderr and trace_stderr to vtraceCap_stderr and trace_stderr,
to signal that they take a va_list (similar to vdebugBelch vs debugBelch).

See #3874 for the original reason behind traceFormatUserMsg.

See the commit msg in #9395 (d360d440) for a discussion about using
null-terminated strings vs strings with an explicit length.

Test Plan:
Run `cabal install ghc-events` and inspect the result of `ghc-events show` on
an eventlog file created with `ghc -eventlog Test.hs` and `./Test +RTS -l`,
where Test.hs contains:

```
import Debug.Trace
main = traceEvent (replicate 510 'a' ++ "bcd") $ return ()
```

Depends on D655.

Reviewers: austin

Reviewed By: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D656

GHC Trac Issues: #8309