Blag dir check.

This fixes #4: do_post (and, from now on, do_*) will first attempt
to move to the root of the blag before doing anything. This is done
by walking up the dir hierarcy until a directory that contains a
.blag file is found. This is safeguarded against symlink bombs or
platform-specific quirks by a counter which, glue.MAX_DIRS_TO_WALK
walked directories later, breaks out of the loop. This isn't
graceful, since the command doesn't actually run, but at least it
doesn't remain stuck forever.

Signed-off-by: Weland Treebark <weland@blinkenshell.org>