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 daemon changes)

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 976