Add restrap step when skipping locker

If we don't build locker, we must take an additional bootstrap step,
so that we can test stability. More precisely, we want to ensure that,
for the given compiler sources, the same bytecode is generated when:

  1. We build a new compiler for given sources with the STARR compiler
    and then use that compiler to build the sources of {library|reflect|compiler}.
  2. We build a new compiler for given sources with the compiler
    that results from building a new compiler with the STARR compiler,
    and then use that compiler to build the sources of {library|reflect|compiler}.
Flip defaults: use released STARR and skip locker.

No longer build locker by default.
The STARR compiler builds the "quick" compiler,
where STARR is a released version of Scala
(specified in the property file `starr.number`).

You can opt out using:

```
locker.enable=1
starr.custom=1
```

The first option enables building locker, using it to build quick.
The second line downloads a custom, unreleased, STARR.

This new behavior used to be opt-in via:

```
locker.skip=1
starr.use.released=1
```
Use Scala 2.11.0-M3 to build the compiler.