Output marking is not thread-safe

I have observed under some conditions, Cabal will interleave stderr and stdout:

```
-----BEGIN CABAL OUTPUT-----
Downloading old-time-1.1.0.3...
[-Extracting /srv/code/haskell-pushbot/cabal-binaries/cabal-testsuite/PackageTests/NewBuild/T4375/cabal.dist/home/.cabal/packages/test-local-repo/old-locale/1.0.0.7/old-locale-1.0.0.7.tar.gz to /srv/code/haskell-pushbot/cabal-binaries/cabal-testsuite/PackageTests/NewBuild/T4375/cabal.dist/work/./dist/tmp/src-28346...-]
-----END CABAL OUTPUT-----
[-Updating old-locale.cabal with the latest revision from the index.-]
writing /srv/code/haskell-pushbot/cabal-binaries/cabal-testsuite/PackageTests/NewBuild/T4375/cabal.dist/home/.cabal/packages/test-local-repo/old-time/1.1.0.3/old-time-1.1.0.3.tar.gz
{+Extracting /srv/code/haskell-pushbot/cabal-binaries/cabal-testsuite/PackageTests/NewBuild/T4375/cabal.dist/home/.cabal/packages/test-local-repo/old-locale/1.0.0.7/old-locale-1.0.0.7.tar.gz to /srv/code/haskell-pushbot/cabal-binaries/cabal-testsuite/PackageTests/NewBuild/T4375/cabal.dist/work/./dist/tmp/src-28736...+}
{+Updating old-locale.cabal with the latest revision from the index.+}
creating dist
```

(in this case, the Downloading message is occurring in a separate thread onto stdout, while "Extracting..." is stderr). This is bad juju. Probably the correct way to fix this is to prefix the markers on every line so we never get confused, but then we have to make sure stderr doesn't get interleaved within a single line this way.

Related: #4357