Fix gsutil update with paths containing whitespace.

gsutil was using os.system invocations to shell out to find, xargs and
chmod in order to ensure that permissions are set correctly on the new
files/directories created inside GSUTIL_DIR during the update process.

These commands don't work if GSUTIL_DIR has whitespace, which happens in
my case because I install gsutil to ~/Google\ Drive/. Although the
update still claims to have succeeded, the os.system commands fail and
spew errors to stdout.

This change fixes the problem by using os.walk and os.chmod instead.

I tested it by forcing an update (editing VERSION) and ensuring that the
commands ran correctly without generating errors.