Transform the database_dir/database_root arg into an absolute path.

adu --create failed badly if called with --database_dir or
--database_root being relative path. This patch fixes the bug.

Unfortunately, it's not completely trivial to obtain the cwd
in a portable and secure manner. The method used in the
new absolute_path() function, while still not bullet-proof, is
the best what one can do if portability is an issue.