Make `FROM scratch` a special cased 'no-base' spec

Make `FROM scratch` a special cased 'no-base' spec

There has been a lot of discussion (issues 4242 and 5262) about making
`FROM scratch` either a special case or making `FROM` optional, implying
starting from an empty file system.

This patch makes the build command `FROM scratch` special cased from now on
and if used does not pull/set the the initial layer of the build to the ancient
image ID (511136ea..) but instead marks the build as having no base image. The
next command in the dockerfile will create an image with a parent image ID of "".
This means every image ever can now use one fewer layer!

This also makes the image name `scratch` a reserved name by the TagStore. You
will not be able to tag an image with this name from now on. If any users
currently have an image tagged as `scratch`, they will still be able to use that
image, but will not be able to tag a new image with that name.

Goodbye '511136ea3c5a64f264b78b5433614aec563103b4d4702f3ba7d4d2698e22c158',
it was nice knowing you.

Fixes #4242

This is a breaking change, isn't it?

Today, the following is perfectly legal:
```Dockerfile
FROM busybox
RUN true
RUN true
FROM busybox
RUN echo hi
```