env and env_init: Eliminate weird auto-initialization of globals.

Merge the two files into env, and make each command explicitly call the
function that sets it up in the way that's needed for that command.

This means we can finally just import all the modules at the top of
each file, without worrying about import order.  Phew.

While we're here, remove the weird auto-appending-'all'-to-targets
feature in env.init().  Instead, do it explicitly, and only from redo and
redo-ifchange, only if is_toplevel and no other targets are given.

redo/jobserver.py

@@ -343,7 +343,7 @@ def ensure_token_or_cheat(reason, cheatfunc):
         if not has_token():
             assert _mytokens == 0
             n = cheatfunc()
-            _debug('%s: %s: cheat = %d\n' % (env.TARGET, reason, n))
+            _debug('%s: %s: cheat = %d\n' % (env.v.TARGET, reason, n))
             if n > 0:
                 _mytokens += n
                 _cheats += n