b=21759 Miscellaneous build fixes

b=21759 Miscellaneous build fixes

The message string given to fatal() cannot be slit with line continuations
as you would strings elsewhere -- for whatever reason.  So let's just un-
split them for now.
Coding style fixups.
Adds a "--set-var" option to lbuild to set/override an environment variable.
This is mainly meant for lbuild testers.
Fix missing - from tar so that the --exclude parameters will be honoured.
Some stderr->stdout redirections to get output into the correct log.