This allows context_by_id() in the master to succeed in returning a
Context with a .name matching the context's name, needed for correct
logging.

Previously this would have logged the empty string, because the master
had no mechanism to know the name of a context created by a child.

Mostly just to avoid embarrassing function size, but it may come in
useful for testing later.

This is a partial fix to a general problem: deciding which bits of state
to keep from the parent, and which to clear out. When forking from a
heavily threaded process, there will be 2x$n_threads fds just sitting
around doing nothing, due to Latch use in the parent.

We can't just close all nonstandard fds post-fork, since user code may
be expecting some FDs to be preserved.

This permits graceful shutdown of individual contexts, without tearing
down everything.

Update mitogen.parent.Stream to also wait for the child to exit, to
prevent the buildup of zombie processes. This introduces a blocking wait
for process exit on the Broker thread, let's see if we can get away with
it. Chances are reasonable that it'll cause needless hangs on heavily
loaded machines.

The Context and Router APIs for constructing children and making
function calls should be available in every parent context, as user code
wants to have access to the same API.

nested_test was failing due to the recent change to centralize
O_CLOEXEC, since stdout and stderr were being marked as non-inheritable.
That meant child processes would start with no stdout/stderr, triggering
a race between Waker opening its pipes, and IoLogger dup2'ing its pipes
over the stdio handles.

Since the stdio handles were closed, Waker would receive one of them as
one end of its pipe, and consequently have it overwritten by IoLogger.

When IoLogger dups over the top of fd 2, it becomes possible for
Waker.on_read() to be called due to pipe's other end to be closed,
causing an OSError exception with errno EAGAIN to appear.

Any long-running system call may suffer EINTR, so arrange for all IO
calls to be wrapped in the restart loop.

This eliminates Context.on_disconnect() and instead moves its
functionality to a signal wired up by ExternalContext.main().

It leaves mitogen.master.Context is in a better condition to move into
mitogen.parent where it belongs.

* Split setup_globals() from setup_package() and make package setup
  optional (fork never needs it -- synthetic package already exists in
  children and the real package exists in masters).

* Add main() parameter to allow passing in the existing Importer
  instance. In forks from children, this means we inherit all the cached
  module state along with the __loader__ used to import any existing
  modules.

This is a hacky layering violation, but it seems the simplest approach
for now: fork needs access to Router, in order to recover the existing
Importer instance.

notice_stream() does that already.

Now it's handled in Side() constructor, it can disappear elsewhere.

This abstracts the pattern found in parent.ModuleForwarder and to a
lesser degree master.ModuleResponser. We can probably use it in those
contexts later.

Not sure why this wasn't done before, seems it should have always been
this way, and can't see any reason it wasn't. Without it, many fds are
leaked into at least .local() children. Closes #163.

It's entirely unclear how test_aborted_on_local_context_disconnect ever
passed, but it was broken by the previous commit.

* IDs are allocated by the parent responsible for contructing a new
  child, using ALLOCATE_ID to the master as necessary to allocate new ID
  ranges.

* ADD_ROUTE is sent up the tree rather than down. This permits
  construction of the new context to complete concurrent to parent
  contexts learning about its existence. Since all streams are strictly
  ordered, it's not possible for any parent to observe messages from the
  new context prior to arrival of an ADD_ROUTE from the parent notifying
  of its existence.

  If the new context, for example, implements an Ansible async task, its
  parent can start executing that without waiting for any synchronous
  confirmation from any parent or the master.

* Since routes propagate up, it's no longer possible for a plain
  non-parent child to ever receive ADD_ROUTE, so that code can be moved
  out of core.py and into parent.py (-0.2kb compressed).

* Add a .routes attribute to parent.Stream, and respond to disconnection
  signal on the stream by propagating DEL_ROUTE for any ADD_ROUTE ever
  received from that stream.

* Centralize route management in a new parent.RouteMonitor class