R1:e4e5c39457f9

Merge __init_env__.php into __init_script__.php

Summary: There are currently two files, but all scripts require both of them,
which is clearly silly. In the longer term I want to rewrite all of this init
stuff to be more structured (e.g., merge webroot/index.php and __init_script__
better) but this reduces the surface area of the ad-hoc "include files" API we
have now, at least.

Test Plan:
- Grepped for __init_env__.php (no hits)
- Ran a unit test (to test unit changes)
- Ran a daemon (to test…