[build2] Optional parameter for build scripts

[Boris Kolpackov]

I've implemented this functionality and it's available in the latest
stage. Quoting the man page:

--structured-result
    Write the result of execution in a structured form. In this mode, instead
    of printing to STDERR diagnostics messages about the outcome of executing
    actions on targets, the driver writes to STDOUT a structured result
    description one line per the buildspec action/target pair. Each line has
    the following format:

    <state> <meta-operation> <operation> <target>

    Where <state> can be one of 'unchanged', 'changed', or 'failed'. If the
    action is a pre or post operation, then the outer operation is specified
    in parenthesis. For example:

    unchanged perform update(test) /tmp/dir{hello/}
    changed perform test /tmp/dir{hello/}

    Currently only the perform meta-operation supports the structured result
    output.

Let me know if there are any issues.