Fix encoding problems in output of scaladoc -help

From the output of `scaladoc -help`:

```
  -doc-source-url <url>                       A URL pattern used to
  build links to template sources; use variables, for example:
  ?{TPL_NAME} ('Seq'), ?{TPL_OWNER} ('scala.collection'),
  ?{FILE_PATH} ('scala/collection/Seq')
```

The ? should be €. Fix that in the source, using the correct
encoding (which is already UTF-8). This requires the *default JVM
encoding* to be UTF-8 when the script is run, that is, passing
`-Dfile.encoding=UTF-8`.

So, after this commit, the euro sign is garbled in the output of
`./qbin/scaladoc -help` and correctly displayed in the output of
`./qbin/scaladoc -Dfile.encoding=UTF-8 -help`.

The next step is to make that option the default, but of course that
change has more potential for breakage.

The encoding problem was introduced in 2011 by commit
2621ee63285808785159a3c24c9e5a5a723b8b9c, and seems to not have been
reported till now.
Scala tools: Set default IO encoding to UTF-8

This changes the template for tool runners to pass
`-Dfile.encoding=UTF-8`. The necessity was uncovered by previous commit.

Alternatively, `-Dfile.encoding=UTF-8` might need to be part of the
local setup - in which case the previous commit is sufficient to fix
Scaladoc's bug.

This is mostly a request for comments; I suppose we want this
change (eventually), and this is the simplest way to change the default
encoding, but I'm not sure it's robust enough.

Moreover, it will break on non-UTF-8 consoles. But since the only
non-ASCII console output seems to be three characters in `scaladoc
-help`'s output, and they've been broken for everybody for two years,
this *might* be acceptable.

The setting is currently not passed again as argument to Scala, and is
not overridable by the user.

My testing was:

* run:
```sh
rm build/quick/bin.complete
ant quick.bin
./qbin/scaladoc -help
```

* observe the output is now not garbled.

I suppose this does not affect the actual input or output of Scala
tools, since they have a separate `-encoding` option to specify that.

The testing I've done is clearly insufficient, and I fear the testsuite
will also not test this aspect (since launchers are usually bypassed).
Testing on Windows is also necessary.