This is used by the platform to disable pushing explicitly in
the case that we're falling back to not using user namespaces, in
which case we dont entirely trust the artifacts we create.

Part of the fix for #92

Here we check if `bwrap --ro-bind / / --unshare-user --uid 0 --gid 0 whoami`
returns successfully and prints 'root', to check if we are indeed capable
of creating user namespaces on the host.

If we are unable, then we save that state in the platform allowing
buildstream to behave differently, and print an informative startup
warning about this.

This fixes issue #92

Dont use the plugin which the message originates from, use the
plugin associated with the child task.

We avoid printing regular start/success/status messages etc
at startup time in advance of the heading, this would introduce
a lot of noise while interrogating source plugins for consistency
state.

This patch holds on to important errors and warnings which might
be raised by core components at startup time, and prints them
immediately after printing the heading.

Attempting to compose a list property would result in an unhandled
exception:

    AttributeError: 'CommentedSeq' object has no attribute 'get'

We now at least detect the situation and produce an exception that
the frontend will report neatly:

    Error loading pipeline: element.bst [line 11 column 4]: Only values of type 'dict' can be composed.

I was getting this error from an attempt to conditionally extend the
sources list:

    sources:
      (?):
        arch == "x86_64":
          - url: http://example.com/x86_64

The correct way to do this is to move the conditional into the parent
dict, e.g.:

    (?):
      arch == "x86_64":
        sources:
          - url: https://example.com/x86_64

It would be nice if the error message could hint at how the user can do
what they want, but it doesn't seem possible in this case.

Profiling suggested that the cumulative time spent in os.path.relpath()
was the dominant cost of utils.list_relative_paths().

Try to call this only once per directory walked.

In Python 3.5, we can optimise further by using os.scandir() and
maintaining relative paths as we go.