Fix a MyPy warning by only passing lists to select.select(). At least on
Python 2.x, select.select() was internally converting the sets to lists
anyway.

By the time lists become inefficient here, it is likely that
select.select() itself will also be inefficient, and need replaced with
.poll() or similar.

No discernible performance different when transferring django.db.models
to a local VM.

* Children should never generate a request for a module that has already
  been sent, however there are a variety of edge cases where, e.g.
  asynchronous calls are made into unloaded modules in a set of
  children, causing those children to request modules (and deps) in a
  different order, which might break deduplication. So add a warning to
  catch when this happens, so we can figure out how to handle it.
  Meanwhile it's only a warning since in the worst case, this just adds
  needless latency.

* Don't bother treating sent packages separately, there doesn't seem to
  be any need for this (after docs are updated to match how preloading
  actually works now).

Overwriting 'fullname' variable caused basically nonsensical filtering.
Result was including the module being searched in the list of
dependencies, which was causing ModuleResponder to send it early, which
was causing contexts to start importing the module before preloading of
dependencies had completed.

* SIGTERM safety net prevents profiler from writing results, so disable
  it when profiling is active.
* fix warning corrupting stream when profiling=True

Knocks 4kb off network footprint for a proxy connection.

Previously we'd send just None in GET_MODULE reply, but now since there
is no single request-reply structure, we must include the fullname in
the LOAD_MODULE response and make all of its data fields None to
indicate the same.

Doesn't yet implement the rules in the docs, but I think the doc rules
could maybe change to match this. Needs lots of cleanup work and
thorough testing, but this is a great start.

* Don't implement the rules for when preloading occurs yet
* Don't attempt to streamily preload modules downstream while this
  context hasn't yet received the final module. There is quite
  significant latency buried in here, but for now it's a lot of work to
  fix.

This works well enough to handle at least the mitogen package, but it's
likely broken for anything bigger.

For gevent, just as in 5f7633cd569f671750ea7f35f6ca79118b345205

It seems gevent automatically sets blocking behaviour on fds produced by
the socket module, which causes the Python process we fork to fail
horribly. So in the child, always reset the blocking flag.

..and enable privileged=True since that's needed for ptrace()

Closes #70

Either execvp succeeds, in which case the process image is replaced, or
execvp throws an exception, in which case control flow exits the frame
anyway.