Use weak but dependency-free entropy source in PhutilOpaqueEnvelope

Summary:
If PHP doesn't have access to /dev/urandom, we currently fatal here before we can get to the setup check for it.

To avoid this, use a weaker random source. This doesn't need to be a strong random source, since it's only protecting against accidental disclosure through logs, etc.

Test Plan:
  - Dumped the resulting key and verified it "looked" random.
  - Dumped a bunch of them to a file and gzipped it, verified it got larger.
  - Had user in question apply patch and verified he got to the /dev/urandom setup check ("open_basedir configured crazy").

Reviewers: vrana, btrahan, asherkin

Reviewed By: asherkin

CC: aran

Differential Revision: https://secure.phabricator.com/D4561