lockfile: Be more forgiving about lockfiles for symlinks.

lockfile: Be more forgiving about lockfiles for symlinks.

As the database is being transitioned from /etc to /var, there is a symlink
from the old to the new location for the database and a symlink for its
lockfile.  This works OK, but it would be more user-friendly to still work
correctly in case the symlink for the lockfile isn't there (since its
existence is non-obvious), so this commit implements that behavior.

Signed-off-by: Ben Pfaff <blp@nicira.com>