This commit only _moves_ code.

Now we are ready to move code from choose_random_entry_impl() to it.

The variable was useless since it was only toggled off in disabled code.

If the 'exit_family' smartlist is empty, we don't consider exit family
anyway.

This code mis-handled the case where a circuit got the same circuit
ID in both directions.  I found three instances of it in the
codebase, by grepping for [pn]_circ_id.

Because of the issue in command_process_relay_cell(), this would
have made roughly one circuit in a million completely nonfunctional.

Fixes bug 12195.

Patch from Roger; changes file by me.

Fixes 10405; bugfix on 0.2.3.9-alpha, where DisableNetwork was
introduced.

On some profiles of Andrea's from #11332, I found that a great deal
of time can still be attributed to functions called from
update_router_have_minimum_dir_info().  This is making our
digestmap, tor_memeq, and siphash functions take a much bigger
portion of runtime than they really should.

If we're calling update_router_have_minimum_dir_info() too often,
that's because we're calling router_dir_info_changed() too often.
And it looks like most of the callers of router_dir_info_changed()
are coming as tail-calls from router_set_status() as invoked by
channel_do_open_actions().

But we don't need to call router_dir_info_changed() so much!  (I'm
not quite sure we need to call it from here at all, but...) Surely
we don't need to call it from router_set_status when the router's
status has not actually changed.

This patch makes us call router_dir_info_changed() from
router_set_status only when we are changing the router's status.

Fix for bug 12170.  This is leftover from our fix back in 273ee3e8
in 0.1.2.1-alpha, where we started caching the value of
update_router_have_minimum_dir_info().

tor_memeq has started to show up on profiles, and this is one of the
most frequent callers of that function, appearing as it does on every
cell handled for entry or exit.

59f9097d introduced tor_memneq here;
it went into Tor 0.2.1.31.  Fixes part of 12169.

I've copied the entries from changes/, labeled the ones that also
appeared in 0.2.4.22, sorted them lightly with a python script
(added to maint), and combined sections with the same name.

I didn't combine sections without a description (e.g. "Minor
bugfixes:"), since we'll probably add a description to those.

fixes bug 12139; bugfix on 0.2.5.1-alpha

Fix for 12115; bugfix on 0.2.5.1-alpha

fixes 12114; bug not in any release.

Improves fix for 12028

(This is how I found out I was trying to test with a kernel too old
for seccomp. I think.)

This is a minimal set of changes for compilation; I need a more
recent kernel to test this stuff.

Bugfix on cfd0ee51; bug not in any released version of tor

Without this fix, when running with bridges, we would try fetching
directory info far too early, and have up to a 60 second delay if we
started with bridge descriptors available.

Fixes bug 11965. Fix on 0.2.3.6-alpha, arma thinks.

fixes part of 12064

There was too much code duplication in doing it the old way, and I
nearly made a copy-and-paste error in the last commit.

fixes part of 12064