l3build: `unpack` fails for a module if unpackdir does not exist

this affects a couple of other commands too.

mkdir is fine to run without checking the dir is needed
since its definition either doesn't care (*NIX) or does
the check already (Windows)

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5117 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: some more work on the doc

* adding a "standalone" script example
* still haven't documented overloading main(), but I might give that a miss for now

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5118 de43f980-851b-0410-b2f7-c40aca1f87e0